Journal of Energy & Natural Resources Law

ISSN: 0264-6811 (Print) 2376-4538 (Online) Journal homepage: http://www.tandfonline.com/loi/rnrl20

Climate change caused by human activities is

happening and it already has major consequences

Kevin E Trenberth

To cite this article: Kevin E Trenberth (2018): Climate change caused by human activities is
happening and it already has major consequences, Journal of Energy & Natural Resources Law,
DOI: 10.1080/02646811.2018.1450895

To link to this article: https://doi.org/10.1080/02646811.2018.1450895

Published online: 01 Jun 2018.

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Journal of Energy & Natural Resources Law, 2018
https://doi.org/10.1080/02646811.2018.1450895

Climate change caused by human activities is happening

and it already has major consequences
Kevin E Trenberth is a distinguished senior scientist at the National Center for Atmospheric
Research*, Boulder, Colorado, US, in the Climate Analysis Section. He has been a leader in the
Intergovernmental Panel on Climate Change and the World Climate Research Programme. He
is one of the most highly cited climate scientists. Email: trenbert@ucar.edu.
(Received 25 February 2018; final version received 1 March 2018)

The climate varies on multiple timescales, but now humans are the main agents of
change and are likely to remain so for the next few centuries. It is generally
understood that human-induced climate change causes global warming, but what
is not adequately appreciated are the direct influences on heavy rainfalls, drought
and storms, at great cost to society and the environment. Although the climate
change effects are modest, perhaps five to 15 per cent for these events, once
thresholds are crossed, things break and damage increases non-linearly. These
aspects are not properly factored into costs of climate change, and preparation
for expected effects is woefully inadequate, exacerbating damage.
Keywords: climate change; extreme weather; mitigation; adaptation; global
warming

1. Introduction
There are many facts related to climate (see below) to demonstrate conclusively that the
problem of human-induced climate change is real. The observational evidence combined
with physical understanding based on well-established physical principles makes this
abundantly clear. Former United States Senator Patrick Daniel Moynihan famously
said ‘Everyone is entitled to his own opinion, but not his own facts.’ The observations
and data – the facts – are of mixed quality and duration, but together tell a compelling
story that leaves no doubt about the human role in climate change. Changes in some
phenomena, such as hurricanes and tornadoes, are confounded by the way observations
are made (eg, the role of satellites) and shortness of reliable records. But the absence of
evidence is not evidence of absence of important changes, and our physical understand-
ing and climate modelling can fill the gaps. However, the facts are not enough. The role of
scientists is to lay out the facts, evidence, prospects and consequences, but the decisions
on what to do about them reside in the realm of politics and should involve all of society.
The Intergovernmental Panel on Climate Change (IPCC),1 US national assess-
ments,2 reports from the National Academy of Sciences3 and many other scientific

*The National Center for Atmospheric Research is sponsored by the National Science Foundation.
1 IPCC, Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (A
Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change, CB Field
and others eds, Cambridge University Press 2012); IPCC, Climate Change 2013: The Physical Science
Basis (TF Stocker and others eds, Cambridge University Press 2013).
2 US Global Change Research Program (USGCRP), Climate Science Special Report: Fourth National
Climate Assessment, Vol I (DJ Wuebbles and others eds, US Global Change Research Program 2017).
3 National Academies of Sciences, Engineering, and Medicine, Attribution of Extreme Weather Events in
the Context of Climate Change (The National Academies Press 2016).

© 2018 International Bar Association

2 KE Trenberth

organisations have proclaimed that ‘global warming is unequivocal’ and it is mainly

caused by human activities. Yet the public is not alarmed. Many politicians either do
not believe in global warming or discount it. But it is not a matter of belief. From
the scientific standpoint, by the time the problems associated with climate change are
so blatant, it will be far too late to do anything about it. Already, the costs are substantial
every year – from tens to hundreds of billions of dollars – from drought, wildfires,
floods, heatwaves, storm surges, hurricanes and strife. The climate events that cause
the damage are isolated events, regional in nature, and affect but few at a time. The
public does not see an integrated view. A major report comes out and it is a headline
for at most one day. But the problem continues, and in fact gets worse every day.
Yet it is no longer news because it remains the same problem, although the problem
has not been solved. It is easy for the public to set it aside.
Climate change is inherently an inter-generational problem. What kind of a
planet are we leaving our grandchildren? It is also a problem of equity among
nations. Small island states and developing countries have not contributed much
to the problem but are affected by it.4 Costs of climate change and air pollution
are often not borne by those who cause these problems. There are substantial uncer-
tainties associated with exactly in what form and where climate change effects will
be felt, but the risks are growing. A normal way society deals with risk is by asses-
sing possible impacts, building resilience, preparing for possible adverse outcomes
and taking out insurance. The precautionary principle should come into play. But
society is not doing enough to mitigate the problem or plan for the consequences.
This aspect is discussed more specifically for the recent Atlantic hurricanes of the
summer of 2017.

2. The climate system

The climate system consists of the atmosphere, oceans, land and cryosphere. The
atmosphere is the most volatile component while both atmosphere and oceans are
fluids and combine to produce the predominant patterns of variability in weather
and the water cycle. We live on land, where availability of water depends on rain-
fall, snow-melt, lakes and rivers, while ice is important where it occurs. Weather
and climate extremes happen all of the time, even in an unchanging climate. Yet
these extremes are becoming more frequent and more severe, and the primary
driver is human-induced climate change.5 Indeed, the main way in which climate
change is already affecting, and is likely to continue to affect, human societies
around the world, is through changes in extreme weather events. Here a
summary is provided of the way to think about the relationship between our
oceans and atmosphere, weather and climate change. It is evident that climate
change is increasing many extremes, with major consequences for human society
and the environment.

4 International Bar Association (IBA) Climate Change Justice and Human Rights Task Force, Achieving
Justice and Human Rights in an Era of Climate Disruption (IBA 2014) www.ibanet.org/
PresidentialTaskForceClimateChangeJustice2014Report.aspx accessed 16 February 2018.
5 Stefan Rahmstorf and Dim Coumou, ‘Increase of Extreme Events in a Warming World’ (2011) 108
PNAS 17905.
 Journal of Energy & Natural Resources Law 3

Figure 1. Global mean annual surface temperatures °C (from NOAA) relative to the 20th-century
average, along with the carbon dioxide concentrations (values at right) in parts per million by
volume (ppmv) (from NOAA) based on the Mauna Loa record after 1958 and ice core bubbles of
air prior to then. Estimates of the pre-industrial values for each are also given (updated from Trenberth
and Fasullo).
Note: KE Trenberth and JT Fasullo, ‘An Apparent Hiatus in Global Warming?’ (2013) 1 Earth’s
Future 19.

Activities on Earth are adjusted to our current climate, which has been relatively
stable throughout the past 2,000 years, coinciding with much of the development of
human civilisation. Natural changes in the past have occurred from small variations
in the sun and effects of volcanic eruptions, and on geological timescales from
changes in the Earth’s orbit around the sun. Now changes in atmospheric composition
from human activities, primarily the burning of fossil fuels and deforestation, are the
main cause of anthropogenic climate change by enhancing the greenhouse effect
(Figure 1). Globally, on a day-to-day basis, the effects from these human activities
are responsible for only about one per cent of the flow of natural energy through the
climate system. However, because anthropogenic global warming is always heating
the planet, excess energy accumulates, and those cumulative effects create a much
bigger impact. The net energy imbalance of the planet is close to 1 W m−2, for all
510 × 1012 m2 of the planet, and so the net heating amount is 0.5 PetaWatts.6 For com-
parison, the total world’s energy consumption in 2015 is estimated as 575 quadrillion
(1015) British thermal units,7 which is 0.02 PW. In the first place, this demonstrates that
the direct human influence is small and cannot compete with that of the sun. In the
second place, the main way humans have an effect is by interfering with the natural
flow of energy through the climate system. However, because of the ‘concrete

6 KE Trenberth, JT Fasullo and J Kiehl, ‘Earth’s Global Energy Budget’ (2009) 90 Bull Amer Meteor Soc
311.
7 US Energy Information Administration www.eia.gov/outlooks/ieo accessed 16 Feb 2018.
4 KE Trenberth

Figure 2. Global OCH for the top 2,000 m of the ocean, relative to the mean for 1961–2010 in 108
Joules per square metre (updated from Cheng and others).
Note: L Cheng and others, ‘Improved Estimates of Ocean Heat Content from 1960–2015’ (2017) 3(3)
Sci Adv e1601545 http://advances.sciencemag.org/content/3/3/e1601545.

jungle’ and a lot of space heating concentrated in cities, there are important local ‘urban
heat island’ effects.
Because this heating is always in the same direction, while it is not large enough to
be important at any instant, it accumulates. Over 92 per cent of it accumulates in the
ocean, which was the warmest ever on record for the globe down to 2,000 m depth
in 2017 (Figure 2). On land, the effects are mitigated in general by water through eva-
porative cooling, and it is mainly in drought where the accumulated effects mount,
exacerbate the drought and greatly increase the risk of wildfire. For instance, over
one month (720 hours) without rain, the accumulated energy of a 1 W m−2 energy
imbalance is equivalent to the full power of a small microwave oven (720 W)
running in every square metre for one hour. No wonder things catch fire!
Importantly, all weather events are now occurring in an environment that has
changed in significant ways, as compared to 50 years ago. The main way this is man-
ifested, the ‘memory’ of these changes, is through the accumulated warming of the
oceans and the loss of Arctic sea ice. Owing to anthropogenic global warming, sea
surface temperatures have warmed by over 1°F since the 1970s, and over the oceans
this has led to five to ten per cent more water vapour in the atmosphere. The warmer
and moister atmosphere in turn has likely led to at least a five to 20 per cent effect
on storms that is greatly exaggerated for extreme weather events. When climate
change’s increased effect on storms is compounded with natural variations, such as
an El Niño event, the effects are much larger and more destructive.
Up until recently, scientists claimed we could not attribute any single weather event
to global warming (climate change) even though the event was consistent with expec-
tations. The reason scientists were reluctant to attribute a single event to global
warming is that weather events cannot be predicted more than about two weeks in
advance, at best; see Section 5 for details. But climate change clearly increases the
odds of such events occurring. In reality, all weather-related events have both natural
and anthropogenic components in this era of climate change. When anthropogenic
climate change and natural climate patterns work synergistically, thresholds are
crossed, records are broken and it can be said that such extreme events would have
 Journal of Energy & Natural Resources Law 5

been very unlikely to occur without global warming. If business-as-usual greenhouse

gas emissions continue, the security and lives of young people and future generations
will be increasingly threatened by ever more extreme weather events than those already
being experienced.

3. The climate is changing

Human activities have led to the release of carbon dioxide and other heat-trapping
‘greenhouse’ gases in sufficient quantity to change the composition of the atmosphere,
resulting in an accumulation of heat in the Earth’s system, commonly referred to as
‘global warming’. The Earth’s climate has responded through higher temperatures in
the atmosphere, land and ocean, ice melting, rising sea level and increases in
extreme weather events (heatwaves, wildfires, heavy rains and flooding). The calendar
year 2016 is by far the warmest on record for the global mean surface temperatures
(GMSTs) (Figure 1). It easily beat out 2015, which in turn beat out the previous
record holder: 2014. Meanwhile, 2017 is now ranked third (Figure 1) (or second,
depending on data set). There is no doubt whatsoever that the planet is warming and
it has major consequences for other aspects of climate. However, there is also consider-
able natural variability manifested in the GMST record; the biggest fluctuations from
year to year are associated with El Niño events. Decadal variations led to a pause in
warming from 2000 to 2013.8 A major El Niño from 2015 to 2016 somewhat inflated
the GMST values, and 2017 values dropped slightly as a result.
The overall warming is caused by human activities, mostly through the changes
in composition of the atmosphere through burning of fossil fuels (eg, industry, elec-
tricity generation, driving cars, flying airplanes, space heating, etc) and deforesta-
tion. Carbon dioxide concentrations in the atmosphere have increased by well
over 40 per cent since pre-industrial times (see Figure 1) and a key reason is that
carbon dioxide has a lifetime of centuries. Air pollution in the form of aerosol par-
ticulates also plays an important role, but because these particulates are washed out
by rainfall, their average lifetime is of the order of one week. Hence their effects are
not global but rather regional, and their production has to continue for their effects
to be present. Their effects are also complex because some reflect the sun and cause
cooling, some (carbonaceous) are dark and absorb the sun’s rays, and many become
involved in clouds and affect the brightness, lifetime and disposition of clouds; in
general, they cause a cooling effect. In contrast, even if we stopped emitting
carbon dioxide into the atmosphere today, the elevated concentrations already estab-
lished would persist for some time, thus underscoring the need for urgent reductions
in carbon dioxide emissions. Hence, changes in atmospheric composition, and par-
ticularly the increase in carbon dioxide concentrations, enhance the greenhouse
effect, although with important regional effects from aerosol particulates. That
global warming is driven primarily from the rise in carbon dioxide can be readily

8 KE Trenberth and JT Fasullo, ‘An Apparent Hiatus in Global Warming?’ (2013) 1 Earth’s Future 19; KE
Trenberth and others, ‘Seasonal Aspects of the Recent Pause in Surface Warming’ (2014) 4 Nature
Climate Change 911 http://rdcu.be/o7wB; KE Trenberth, ‘Has There Been a Hiatus?’ (2015) 349
Science 691.
6 KE Trenberth

Figure 3. Global sea level rise based upon altimeter measurements from space since late 1992, with
the annual cycle removed (adapted and updated from Nerem and others).
Note: RS Nerem and others, ‘Estimating Mean Sea Level Change from the TOPEX and Jason Alti-
meter Missions’ (2010) 33(supp 1) Marine Geodesy 435; http://sealevel.colorado.edu.

demonstrated by using comprehensive climate models, which enables us to also

make projections into the future.9
Most of the energy imbalance as excess heat, over 90 per cent, ends up in the
ocean.10 Hence, the oceans are warmer, Arctic sea ice is melting, and land glaciers
and ice sheets, such as Greenland, are also melting. The largest temperature rises are
occurring in the Arctic, where bright reflective snow and ice are melting to reveal
dark ocean and land. This darkened surface reflects less sunshine, compounding the
warming that is causing the melting in the first instance. The combination of a
warmer ocean that expands and extra melt-water in the oceans means that sea level
is rising at a rate of well over a foot per century (Figure 3).
Figures 2 and 3 show that ocean heat content (OHC) and sea level are clearly rising
as the planet warms, and the noise level of natural variability is a lot less than for
GMST11 (Figure 1).

4. How global warming affects extreme events

As well as the overall heating of the planet mainly from human-induced changes in the
atmospheric composition, which leads to general temperature increases in the

9 IPCC (n 2).
10 K von Schuckmann and others, ‘An Imperative to Monitor Earth’s Energy Imbalance’ (2016) 6 Nature
Climate Change 138.
11 L Cheng and others, ‘Taking the Pulse of the Planet’ (2018) 99, Eos, doi:10.1029/2017EO081839,14.
 Journal of Energy & Natural Resources Law 7

atmosphere and oceans and melting ice, there are substantial impacts on extreme events.
Indeed, the biggest impacts of climate change on society and the environment arise from
changes in extremes. These are realised through the daily weather systems, which natu-
rally produce tremendous variability on all timescales and over many different spatial
scales. Hence just by chance, extreme values of temperatures, precipitation or wind,
and so forth, occur from vigorous weather systems. With global warming, some of
these extremes are pushed higher and beyond previous values, creating new records.
Moreover, global warming often pushes values over various thresholds used for design
purposes: whether for heat, rain, wind or sea level, and accordingly things break. This
also means that the events and the new records are episodic. There is not a continuous
level of high values, rather the values fluctuate substantially as they have always done
with natural weather patterns. It also means that, in one month, records are broken at
one location while, in the next month, records break somewhere else, and then some-
where else again. The fact that the extremes occur in different places over time means
that the public often does not connect them to climate change, and their accumulated
effects have been greatly underestimated by many. It also means that because of the
natural climate variability from year to year, it is often difficult to detect conclusively
the climate change influences – an issue of signal-to-noise, as discussed later.
The conceptual framework for how extremes change with climate change is given in
Figure 4.12

4.1. Heatwaves
The most obvious expectation is for an increase in short-duration heatwaves and their
impacts as overall temperatures rise. Often, these result in temperature rises beyond
anything previously experienced in recorded history, and this has been borne out in
many studies;13 see also IPCC reports14 and other assessments.15 Heatwaves nearly
always occur in association with a strong, slow-moving anticyclone. The major
European heatwave in the summer of 200316 was one of the first to be well documented
both in terms of its detection as extremely unusual, and in respect of its attribution to
anthropogenic climate change using climate models. There were major consequences in
terms of wildfires and loss of life. A more recent example is the extreme Russian heat-
wave of 2010, again with widespread wildfires, smoke, agricultural losses and loss of
life. Some confusion and debate has occurred in the scientific literature about this event
over the cause and rarity of the weather situation, versus the role of human-induced

12 KE Trenberth, ‘Atmospheric Moisture Residence Times and Cycling: Implications for Rainfall Rates
and Climate Change’ (1998) 39 Climatic Change 667; KE Trenberth, ‘Conceptual Framework for
Changes of Extremes of the Hydrological Cycle with Climate Change’ (1999) 42 Climatic Change
327; KE Trenberth and others, ‘The Changing Character of Precipitation’ (2003) 84 Bull Amer
Meteor Soc 1205; KE Trenberth, ‘Changes in Precipitation with Climate Change’ (2011) 47 Climate
Research 123.
13 GA Meehl and C Tebaldi, ‘More Intense, More Frequent, and Longer Lasting Heat Waves in the 21st
Century’ (2004) 305 Science 994; SM Papalexiou and others, ‘Global, Regional, and Megacity
Trends in the Highest Temperature of the Year: Diagnostics and Evidence for Accelerating Trends’
(2018) 6 Earth’s Future 71.
14 IPCC (n 1).
15 US Global Change Research Program (USGCRP) (n 2).
16 P Ciais and others, ‘Europe-Wide Reduction in Primary Productivity Caused by the Heat and Drought in
2003’ (2005) 437 Nature 529.
8 KE Trenberth

Figure 4. A summary figure of the effect of human-induced climate change via changes in atmos-
pheric composition on extremes of storms and precipitation. Adapted from Trenberth (1998). KE
Trenberth, ‘Atmospheric Moisture Residence Times and Cycling: Implications for Rainfall Rates
and Climate Change’ (1998) 39 Climatic Change 667.
 Journal of Energy & Natural Resources Law 9

warming.17 As discussed in the next section on attribution, this confusion arises

because the weather events (strong anticyclones) tend to occur naturally, while it is
the global warming that pushes what would have been an extreme event anyway
into one that goes well outside previous bounds and causes major strife.
High temperatures can result in detrimental health, economic and social impacts.16
The European 2003 and the Russian 2010 heatwaves caused, respectively, almost
70,000 and 55,000 deaths,18 while an average of 658 deaths were reported annually
during 1999–2009 in the US alone due to excessive heat.19 Extreme high temperatures
may cause human casualties in large cities and have profound impacts on farms due to
reduced crop productivity and adverse effects on animals, including mortality. Tempera-
ture extremes place stress on infrastructure, transportation, water supply and electricity
demand; severely affect ecosystems and forests; and increase wildfire activity. Heat
strokes – the most lethal condition of hyperthermia – can be caused by exposure to
high ambient environmental temperatures.20 More frequent, more intense and longer-
lasting heatwaves are robustly projected in the 21st century as a result of human-
induced global warming.

4.2. Drought and wildfire

In the US, and indeed in mid-latitude continental areas around the world, there is a
strong negative correlation between monthly mean temperatures and precipitation in
the summer half year, as there is year-round in the tropics. Heatwaves, especially
ones of longer duration, often occur in association with drought (see Dai21 for a
general discussion). The anticyclonic conditions that persist in a drought situation
make for dry, settled weather, with little or no precipitation. Under these circumstances,
the land and vegetation dry out, and the modest extra heat from global warming exacer-
bates the dry conditions. Evaporative cooling ceases as plants wilt, wildfire risk
increases and the heat intensifies. That in turn increases the atmospheric demand for
moisture, further drying out the vegetation in a vicious cycle.
The warmest year on record for the US as a whole was 2012, when there was a
widespread drought in association with persistent anticyclonic conditions over much
of the country. Extreme drought was estimated to cover 39 per cent of the country at
its peak in September 2012, rivalling the Dust Bowl years in the early 1930s. According
to the 4 September 2012 drought monitor, 64 per cent of the country was in moderate to
extreme drought. Wildfires became endemic in many places, and firefighting costs

17 R Dole and others, ‘Was There a Basis for Anticipating the 2010 Russian Heat Wave?’ (2011) 38
Geophys Res Lett L06702; FEL Otto and others, ‘Reconciling Two Approaches to Attribution of the
2010 Russian Heat Wave’ (2012) 39 Geophys Res Lett L04702; KE Trenberth and JT Fasullo,
‘Climate Extremes and Climate Change: The Russian Heat Wave and Other Climate Extremes of
2010’ (2012) 117 J Geophys Res D17103.
18 J-M Robine and others, ‘Death Toll Exceeded 70,000 in Europe during the Summer of 2003’ (2008) 331
C R Biol 171.
19 SM Papalexiou and others (n 13).
20 KE Smoyer-Tomic, R Kuhn and A Hudson, ‘Heat Wave Hazards: An Overview of Heat Wave Impacts in
Canada’ (2003) 28 Natural Hazards 465.
21 A Dai, ‘Drought under Global Warming: A Review’ (2011) 2 Wiley Interdisciplinary Reviews: Climate
Change 45.
10 KE Trenberth

soared.22 As a result of these events and the agricultural and livestock losses, the net
cost has been estimated as over $75bn, although a partial accounting by the National
Oceanic and Atmospheric Administration (NOAA) lists it as $32bn.23 Wildfire Today
reports the firefighting costs alone in 2012 were $2bn.24
Perhaps the best example of how climate change can lead to an increase in drought
conditions is in the American West, particularly California.25 A record-setting drought
began in 2012 and persisted until 2016 in spite of the big El Niño event (which favours
more storms coming into the West Coast). It included the lowest annual precipitation on
record, the highest annual temperature, as well as the most extreme drought indicators
ever recorded in California. Along with widespread water shortages, the drought
brought prolonged and costly wildfires. Indeed, wildfires were rampant throughout
the West, especially in the summer of 2015, with wildfires widespread in Alaska,
western Canada, Washington, Oregon and California. In May 2016, a major wildfire
broke out in Fort McMurray, Alberta following five to eight months of prolonged
(El Niño-related) drought. Major wildfires continued again in August 2016 and July
2017 in California, and the consensus has become that the wildfire season in California
is now almost continuous. In early 2017, in association with unusually high sea temp-
eratures in the subtropical North Pacific, the drought in California was abated with
heavy rains and snows, leading to flooding in many areas. This was a boon in terms
of snowpack to the Sierra Nevadas and Rocky Mountains.
Certain bugs and diseases flourish under these warmer and dryer conditions, such as
the bark beetle, which is decimating forests across the West. Increased carbon dioxide is
not good for plants!

4.3. Storms and precipitation

Perhaps less obvious, but even more dangerous than heat, are the effects of a warming
planet on the water cycle, in which the oceans play a key role. The atmosphere holds
about four per cent more moisture per 1°F (or seven per cent per 1°C) increase in temp-
erature, which leads to increased water vapour in the atmosphere, and this provides the
biggest influence on precipitation. It is undisputed that water vapour is a powerful green-
house gas, and hence this amplifies the original warming substantially. In addition, sea
surface temperatures have warmed by more than 1°F since the 1970s, and over the
oceans this has led to five to ten per cent more water vapour in the atmosphere.26

22 CD Allen and others, ‘A Global Overview of Drought and Heat-Induced Tree Mortality Reveals Emer-
ging Climate Change Risks for Forests’ (2010) 259 Forest Ecology and Management 660; JT Abatzo-
gloua and AP Williams, ‘Impact of Anthropogenic Climate Change on Wildfire across Western US
Forests’ (2016) 113 PNAS 11770.
23 www.ncdc.noaa.gov/billions/events.pdf accessed 16 February 2018.
24 http://wildfiretoday.com/2018/03/29/firefighting-costs-1985-2017 accessed 22 May 2018.
25 AP Williams and others, ‘Causes and Implications of Extreme Atmospheric Moisture Demand during the
Record-Breaking 2011 Wildfire Season in the Southwest United States’ (2014) 53 J Applied Meteor Cli-
matol 2671; NS Diffenbaugh, DL Swain and D Touma, ‘Anthropogenic Warming Has Increased
Drought Risk in California’ (2015) 112 PNAS 3931; J Worland, ‘How the California Drought Is Increas-
ing the Potential for Devastating Wildfires’ Time (8 May 2015) http://time.com/3849320/california-
drought-wildfires.
26 KE Trenberth, J Fasullo and L Smith, ‘Trends and Variability in Column-Integrated Atmospheric Water
Vapor’ (2005) 24 Clim Dyn 741.
 Journal of Energy & Natural Resources Law 11

Storms, whether individual thunderstorms, extratropical rain or snow storms, or tro-

pical cyclones and hurricanes, supplied by increased moisture, produce more intense
precipitation events, even in places where total precipitation is decreasing.27 The
increased moisture and related latent heat release can intensify storms and perhaps
double the original change so that the precipitation increases five to 20 per cent. The
effect on the storm depends on where the precipitation and released heat occur relative
to the storm centre. For hurricanes, the effect is direct and the result can be doubled or
more. For extratropical storms the effects are more complicated and the effect is a factor
of one to two and varies from storm to storm.
Nevertheless, it leads to much stronger and more intense rains and snows, and it
increases risk of flooding that exceeds previous bounds for extreme weather events.
At the same time, dry spells in between such events also increase. Indeed, in places
where it is not raining, the extra heat dries things out, exacerbating heatwaves as the
evaporative cooling is lost. Hence, droughts set in quicker and become more intense,
increasing risk of wildfire.28 This is an especially dangerous problem in the US
West, as noted above.
Examples are discussed in more detail below. However, in Colorado, the unprece-
dented widespread flooding along the Front Range in September 2013 is a case in point.
The moisture sources came from very warm ocean regions to the south (the Gulf of
Mexico and especially from west of Mexico) that undoubtedly had a global warming
component.29 More recently, widespread flooding occurred in Missouri (November–
December 2015), in Houston in April 2016, in Louisiana in August 2016, and in the
Carolinas from Hurricane Matthew in October 2016. The major winter storm ‘Jonas’
that ‘bombed’ Washington, DC with several feet of snow in January 2016 is another
example of such an extreme event. Meanwhile, torrential rains, flooding, mud slides
and loss of life occurred in South America: in northern Chile in late February 2017,
in Peru in March and Colombia in early April in association with a coastal El Niño
that led to very high sea temperatures off the Pacific coast in combination with
global warming.
Without climate change, these events would have been properly labelled as ‘one in
1,000-year events’. However, because of climate change and its effects on the environ-
ment, they are no longer one in 1,000-year events, and instead, they are now more likely
one in 50-year or 100-year events. They are still uncommon, but not unlikely.

4.4. Tropical storms and hurricanes

Tropical storms and hurricanes/typhoons mostly occur in the deep tropics in summer in
association with high sea surface temperatures (SSTs) over 27°C. In turn, these reflect
high OHC below the surface and it is this heat energy that is transferred into the atmos-
phere through evaporation, moistening the atmosphere, while evaporative cooling
occurs in the ocean. The fuel for tropical storms and hurricanes comes from the

27 KE Trenberth (n 12).
28 AL Westerling and others, ‘Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity’
(2006) 313 Science 940.
29 KE Trenberth, JT Fasullo and TG Shepherd, ‘Attribution of Climate Extreme Events’ (2015) 5 Nat Clim
Change 725.
12 KE Trenberth

release of the latent heat in heavy rainfall as the moisture is gathered into the storm and
condensed.30
One harmful aspect of hurricanes is the fierce winds that cause destruction to
people’s homes and other buildings and infrastructure. However, hurricanes are also
responsible for huge storm surges in coastal regions that can be very damaging and
are expected to become much worse due to both stronger winds and higher sea
levels. The most widespread damage, though, is actually the flooding from torrential
rains that can extend hundreds of miles from the coast.
One major source of variability in tropical SSTs is the El Niño phenomenon that
produces a warming in the central and eastern Pacific with a corresponding shift in tro-
pical storm activity into that region at the expense of other regions.31 Hurricanes
become more frequent in the eastern North Pacific but decrease in the Atlantic, for
example. Indeed, there is always a competition throughout the tropics for where the
main activity occurs, and high SSTs are the main factor. Once activity is under way
in one region of the tropics, it tends to suppress activity elsewhere by creating a
large overturning circulation in the atmosphere that creates subsiding stable air else-
where and wind-shear in in-between regions (where the low-level winds and upper-
level winds in the troposphere at jet stream level are in different directions and/or
speeds), and this tends to blow a developing vortex apart. Accordingly, tropical
storms are clustered and cannot occur everywhere at once.
In general, climate warming invigorates tropical storm activity by adding energy to
the storms, but it can be manifested in several ways. With climate change, it is expected
that hurricanes will contain heavier rains and become more intense, longer lasting and
possibly larger in size, but fewer in number, as one big storm essentially replaces the
effects of several smaller, weaker storms in terms of the heat energy pulled out of the
ocean. Owing to the large natural climate variability from year to year and unreliable
records prior to the satellite era (∼1980), it is difficult to clearly detect climate change
influences on tropical storm activity. ‘Detection’ relies on a climate signal that is larger
than the noise of natural variability, confounded also in this case by unreliable data.
So, it is not that there is no signal, but rather that the noise is large. Indeed, there is
very compelling evidence that there is a climate signal to increased tropical storm activity.
Examples of increased activity are the record-breaking exceptionally large number
and strength of storms in the Atlantic in 2005, Superstorm Sandy on the East Coast in
2012, the strongest land-falling typhoon on record: Haiyan in 2013 that went through
the Philippines, and the very strong storms recorded in several regions in 2015 and
2016 (strongest in the southern hemisphere – Winston in 2016 that went through
Fiji). Then, in 2017, it was the Atlantic’s turn, with Harvey, Irma and Maria creating
devastation in Texas, Florida and the Caribbean Islands, and Puerto Rico. The year
2015 is the most active year globally for hurricanes/typhoons ever. The latter is in
part because it was an El Niño year, but it highlights the fact that high sea surface

30 KE Trenberth, ‘Warmer Oceans, Stronger Hurricanes’ (2007) (July) Scientific American 45; KE Tren-
berth, CA Davis and J Fasullo, ‘Water and Energy Budgets of Hurricanes: Case Studies of Ivan and
Katrina’(2007) 112 J Geophys Res D23106; KE Trenberth and J Fasullo, ‘Water and Energy Budgets
of Hurricanes and Implications for Climate Change’ (2007) 112 J Geophys Res D23107; KE Trenberth
and J Fasullo, ‘Energy Budgets of Atlantic Hurricanes and Changes from 1970’ (2008) 9 Geochem,
Geophys, Geosyst Q09V08.
31 Ibid.
 Journal of Energy & Natural Resources Law 13

temperatures from whatever reason produce bigger and stronger storms. At the same
time, there are quiet years that highlight the large variability.
Costs of flooding for a number of events have been assigned32 and hurricanes
Katrina, Rita and Wilma in 2005 cost over $180bn (2011 prices). The recent Atlantic
hurricanes in 2017 are estimated to have damages of over $230bn.33

4.5. Snowfall and snow cover

In winter over the northern hemisphere land, the snow season is getting shorter at each
end as more precipitation arrives as rain. Generally, the biggest snowfall occurs with
temperatures just below freezing, and hence, in mid-winter, the prospects, as observed,
are for bigger snowfalls and larger snowpack from November through January. With
climate change, it is no longer ‘too cold to snow’ very often. In contrast, snowpack
is observed to be much reduced across the northern hemisphere from March through
August 1966 to 2014. As a result of global warming, snow-melt starts sooner, run-
off occurs sooner in the spring, and the risk of drought and water shortages is
greater in summer, along with wildfire and insect pest infestations.

5. Attribution
Scientists are working to attribute causes to weather and climate events, which is often
challenging. Owing to the chaotic nature of the atmospheric circulation (often depicted
by the flap of a butterfly’s wings changing the future weather), the detailed day-to-day
weather cannot be forecast more than about two weeks into the future. Many repeated
computer runs with small perturbations in initial states (forming ensembles) are used to
bring out the robust features in future predictions versus those that depend on unknown
details. This is done even for two-week weather forecasts and is essential for climate
simulations and predictions. In dealing with climate predictions, then, the goal is to
predict not the detailed evolution but the general patterns of weather, such as those
that occur from one season to the next. Hence, the reason scientists are reluctant to attri-
bute a single event to global warming is that weather events cannot be predicted more
than about two weeks ahead, but climate change may change the odds of such events
occurring.
In the past, the traditional way of approaching attribution tried to deal with all
aspects of the problem. But the changes in weather phenomena and weather systems,
where they go and so forth have infinite variety (called weather) and any climate
change signal is small (except in the case of the ozone hole). This has confounded
the results.34 In particular, the conventional approach to attribution of climate events
is to characterise the event and ask (i) whether the likelihood or strength of such
events has changed in the observational record; and (ii) whether this change is consist-
ent with the anthropogenic influence as found in one or more climate models. This

32 www.ncdc.noaa.gov/billions/summary-stats accessed 16 February 2018.

33 E Michel-Kerjan and H Kunreuther, ‘Redesigning Flood Insurance’ (2011) 333 Science 408; NOAA,
‘Billion Dollar Weather and Climate Disasters’ (2017) www.ncdc.noaa.gov/billions/events/US/1980-
2017.
34 M Hoerling and others, ‘Northeast Colorado Extreme Rains Interpreted in a Climate Change Context’
(2014) 95 Bull Am Meteorol Soc S15.
14 KE Trenberth

approach has had considerable success with extremes that are strongly governed by
thermodynamic aspects of climate change, especially those related to temperature,
each finding providing another independent line of evidence that anthropogenic
climate change is affecting climate extremes. But the traditional approach requires
many climate model runs with and without climate change present to sort out how
unusual the weather event was and how the odds were changed by climate change.
Because of the infinite natural variety of weather and the often-uncertain nature of
the human influences, such changes are mostly very small and lost in the noise. The
huge computational demand precludes the near real-time commentary required by
the media.
Hence, the conventional approach is severely challenged when evaluating climate
extremes that are strongly governed by atmospheric circulation, including local
aspects of precipitation. It is inherently conservative and prone to false negatives,
which underestimate the true likelihood of the human influence. This is all the more
reason why the new ‘conditional’ approach35 provides more insight and illumination
as to what is going on and the role of climate change. Instead, it is more useful to
regard the extreme circulation regime or weather event as being secondary – it is the
means whereby the event happens – and focus on the effects of the well-established
changes in the environment from global warming on the impacts of the particular
event. Therefore, it is better to examine whether known changes in the climate
system’s thermodynamic state (ie, temperature related) affected the impact of the par-
ticular event. Because the water-holding capacity of the atmosphere depends strongly
on temperature – it increases seven per cent per °C – there is also a direct relationship
with humidity and precipitation. In other words, given the change in atmospheric cir-
culation that brought about the event, how did climate change alter its impacts?
Therefore, a fruitful and robust approach to climate extreme-event attribution is to
regard the circulation regime or weather event as a conditional state (whose change in
likelihood is not assessed) and ask whether the impact of the particular event was
affected by known changes in the climate system’s thermodynamic state (for
example, sea level, sea surface temperature or atmospheric moisture content), concern-
ing which there is a reasonably high level of confidence.
The National Academy of Sciences36 in March 2016 presented both approaches as
two aspects of the same spectrum, virtually without comment. But the message was that
the strongly conditioned approach is completely acceptable, and moreover that the tra-
ditional approach will be limited by adequacy of the modelling tools available. Never-
theless, the large traditional attribution community has been hostile to the new
approach37 and has evidently been threatened by it.
The consequences of climate change are that things dry out quicker (stronger, longer
droughts) – as the atmosphere demands more evaporative moisture – and the extra

35 KE Trenberth, ‘Changes in Precipitation with Climate Change’ (2011) 47 Climate Research 123; KE
Trenberth, ‘Framing the Way to Relate Climate Extremes to Climate Change’ (2012) 115 Climatic
Change 283; KE Trenberth, JT Fasullo and TG Shepherd, ‘Attribution of Climate Extreme Events’
(2015) 5 Nat Clim Change 725.
36 National Academies of Sciences, Engineering, and Medicine, Attribution of Extreme Weather Events in
the Context of Climate Change (The National Academies Press 2016).
37 EA Lloyd and N Oreskes, ‘Climate Change Attribution: When Is It Appropriate to Accept New
Methods?’ (2018) 6 Earth’s Future 311 https://doi.org/10.1002/2017EF000665.
 Journal of Energy & Natural Resources Law 15

moisture means heavier rains and greater risk of flooding elsewhere, so that ironically,
the risks of both extremes of the hydrological cycle are substantially increased. This is
confusing to many people, but of course the floods and droughts occur at different times
or even in different years, and different places at the same time. Those studies that have
sought to understand this through changes in the weather patterns have generally failed
and concluded that natural variability rules. But, as explained above, the weather pat-
terns occur in a different environment, one that is warmer and moister and thus one
where the atmosphere demands more moisture and causes drying where it is not
raining, but one that provides much more moisture to storms with resulting much
heavier rains, or even snows, where it is precipitating.
Of course, there are some observed changes in weather patterns, most notably in the
southern hemisphere in association with the ozone hole, and small changes elsewhere
are projected in the future. In addition, some changes have apparently occurred in
association with decadal variability (eg, related to the pause in the rise of GMST
from 2000 to 2013; Figure 1) to further confound results, but the signal is not that of
climate change. This confusion has been apparent in IPCC reports and national assess-
ments. Below I provide some examples where the thermodynamic aspects are empha-
sised to bring out the human influence.

6. Examples
Super Typhoon Haiyan/Yolanda, November 2013: the strongest recorded storm ever to
reach land. The OHC and sea level in the Philippines region had both increased a great
deal since 1993 and especially since 1998. Consequently, as Typhoon Haiyan
approached the Philippines, it was riding on very high sea surface temperatures
(SSTs) with very deep support through the high OHC. The strong winds and resulting
ocean mixing did not cause as much cooling as would normally be experienced, helping
the storm to maintain its tremendous strength. Moreover, the storm surge was undoubt-
edly exacerbated considerably by the sea levels, which were some 30 cm (one foot)
above 1993 values. Although natural variability played an important role, increased
OHC from the Earth’s energy imbalance (climate change) made the typhoon more
severe.
Superstorm Sandy: Superstorm Sandy struck the Northeast in late October 2012 and
devastated the New Jersey shore and parts of New York City, including flooding the
subway and tunnels to Brooklyn and New Jersey, and 233 lives were lost. Munich
Re puts the cost of the storm surge at $68.5bn although other estimates are higher.38
Because the storm was very well predicted a week ahead of time by sophisticated
numerical weather prediction models, it was possible to run many computer-based fore-
casts with observed SSTs versus those with climatological conditions, which showed
almost no effects on the track of the storm, but large and significant effects for intensity,
wind strength and size.39 Hence, Sandy was undoubtedly larger and stronger as a result

38 www.munichre.com/site/corporate/get/documents_E1564247680/mr/assetpool.shared/Documents/5_
Touch/_NatCatService/Significant-Natural-Catastrophes/2014/10-costliest-hurricanes-ordered-by-
overall-losses.pdf
39 L Magnusson and others, ‘Evaluation of Medium-Range Forecasts for Hurricane Sandy’ (2014) 142
Mon Weather Rev 1962.
16 KE Trenberth

of climate change, and the storm surge was much greater owing to high sea levels and
stronger winds. It is quite likely that the subways and tunnels in and around New York
would not have flooded without the warming-induced increases in sea level and in
storm intensity and size.40
This is an excellent example of thresholds being crossed with highly non-linear con-
sequences. Relatively small increases in water from the climate change component
caused billions of dollars in damage.41
California drought, 2013–16: One study of the recent California drought that
focused on atmospheric circulation effects found no significant trends in winter precipi-
tation in recent decades while another pointed out the critical role of the record-high
annual mean temperatures in combination with record-low annual precipitation for
2013, which led to increased evapotranspiration and more intense drought. Another
study42 suggested that eight to 27 per cent of the warming contributing to the
drought was anthropogenic, but even this is likely an underestimation as it used
inadequate models and did not account for the changing snowpack. The combination
of the weather pattern and climate change had impacts on water shortages, vegetation
and agriculture, and increased wildfire risk. The odds of this combination of events
have increased with human-induced climate change and anthropogenic warming
causing increased risk of drought and heatwaves.43 Again, several studies are consistent
with the view that the atmospheric circulation changes are not the dominant factor, as
they arise mostly from natural reasons, while climate change greatly increases heat and
drying under favourable conditions and thus increases the impacts.
Colorado floods, September 2013: In Colorado, the unprecedented heavy rains
(over nine inches in 24 hours, over 17 inches in several locations from 9 to 15 Septem-
ber) led to widespread flooding along the Front Range causing widespread devastation,
with 345 homes lost and over 550 more damaged. The unusual tropical moisture
sources came from very warm ocean regions to the south (the Gulf of Mexico and
especially from west of Mexico), where twin hurricanes Manuel and Ingrid formed
as soon as the moisture flow to the north was cut off and the double strike in
Mexico led to 192 deaths and nearly $6bn in damage.44 The exceptionally high
SSTs in the absence of an El Niño undoubtedly had a global warming component.
Southeast flooding in 2016 from both Louisiana floods (August) and Hurricane
Matthew (October): In both cases, record-high values of atmospheric moisture were
measured (by instrumented radio-sonde balloons) in association with very high SSTs
in the Gulf of Mexico and in the subtropical North Atlantic. The moisture was trans-
ported into the region of the flooding by the storms and resulted in unprecedented

40 KE Trenberth, JT Fasullo and TG Shepherd, ‘Attribution of Climate Extreme Events’ (2015) 5 Nat Clim
Change 725.
41 For example, on 14 November 2012 the New York Times editorial ‘Money to Rebuild after Sandy’
reported that ‘New York, New Jersey and Connecticut – the states hit hardest by Hurricane Sandy –
will need tens of billions of federal dollars to repair bridges, tunnels, subway and commuter rail lines,
rebuild schools, power stations and homes, and pay off staggering amounts of overtime’ and noted
the request from Mr Cuomo (the Governor) for $30bn.
42 NS Diffenbaugh, DL Swain and D Touma, ‘Anthropogenic Warming Has Increased Drought Risk in
California’ (2015) 112 PNAS 3931.
43 AP Williams and others, ‘Contribution of Anthropogenic Warming to California Drought during 2012–
2014’ (2015) 42 Geophys Res Lett 6819.
44 KE Trenberth, JT Fasullo and TG Shepherd (n 40).
 Journal of Energy & Natural Resources Law 17

rains and flooding. By one estimate, climate change increased the chance of the three-
day torrential rains in south Louisiana by over 40 per cent.45 The impacts were
profound.
Summer 2017 Atlantic hurricanes: Prior to the beginning of northern summer of
2017, OHC was the highest on record both globally and in the Gulf of Mexico, but
the latter sharply decreased with Hurricane Harvey via ocean evaporative cooling.
The lost ocean heat was realised in the atmosphere as moisture, and then as latent
heat in record-breaking heavy rainfalls. Accordingly, record-high ocean heat values
not only increased the fuel to sustain and intensify Harvey, but also increased its flood-
ing rains on land. Harvey could not have produced anything like as much rain without
human-induced climate change. Moreover, proactive planning for the consequences of
human-caused climate change is not happening in many vulnerable areas, making the
disasters much worse. Planning for such supercharged hurricanes (adaptation) by
increasing resilience (eg, better building codes, flood protection, etc) and preparing
for contingencies (such as evacuation routes, power cuts and so forth) is essential
but not adequate in many areas, including Texas, Florida and Puerto Rico where
Harvey, Irma and Maria took their toll.46
These events highlight that it is the combination of natural variability (weather, El
Niño, etc) and climate change that is critical; when they go in the same direction records
are broken. Hence there are more extreme climate events of all sorts. The result is huge
in terms of both economic loss and human suffering.

7. Conclusions
There are increasing numbers of billion-dollar disasters in the US and around the world.
In the US in the past 20 years through 2016, there had been on average over $42bn
incurred in weather-related disaster costs, according to NOAA.47 Figure 5 shows
overall monetary losses worldwide from Munich Re for 1980 through 2016 along
with the contributions from weather and climate-related events in billions of US
dollars. Now 2017 adds another spike to the plot of over $300bn from the hurricanes
and wildfires. A lot of this depends on where the disaster happens and how much infra-
structure is present, and it does not measure the human factors, strife and loss of life,
especially in developing countries.
Extreme weather has always happened, but now thresholds are being crossed,
records broken, and so at least a portion of these losses can be ascribed to climate
change. There is no precise tool for how much should be ascribed to human influ-
ences.48 On the one hand, records can be broken even without climate change. At
the very least, the storm, precipitation and weather-related events are amplified by
water vapour increases of five to ten per cent since 1970, and these lead to five to 20

45 K van der Wiel and others, ‘Rapid Attribution of the August 2016 Flood-Inducing Extreme Precipitation
in South Louisiana to Climate Change’ (2017) 21 Hydrol Earth Syst Sci 897; www.noaa.gov/media-
release/climate-change-increased-chances-of-record-rains-in-louisiana-by-at-least-40-percent.
46 KE Trenberth and others, “Hurricane Harvey links to ocean heat content and climate change adaptation.”
(2018) Earth’s Future. Doi:10.1002/2018EF000825.
47 www.ncdc.noaa.gov/billions/summary-stats accessed 16 February 2018.
48 National Academies of Sciences, Engineering, and Medicine, Valuing Climate Damages: Updating Esti-
mation of the Social Cost of Carbon Dioxide (The National Academies Press 2017).
18 KE Trenberth

Figure 5. Estimates of insured (red) and total (orange) losses from various weather-related phenom-
ena worldwide in billions of US dollars for 1980 through 2016. Based upon data from Munich Re
(downloaded 26 February 2018).

per cent increases in precipitation intensity or more. But, it is not appropriate to then
assign only 5–20 per cent of the cost of the disaster to human-induced climate
change because the damage is highly non-linear.
It is generally accurate to say that extreme events, which break records and cross
thresholds, would not have happened without global warming, because otherwise the
event would have been well within previous experience. Thus, thresholds are crossed
and records are broken because of anthropogenic climate change. Moreover, every
event is different. Events occur in different places and evolve very differently,
whether floods, wildfires or heatwaves, but they all have one aspect in common:
they would not have been as severe without the human influence. In light of this,
one could argue that the whole cost might be assigned to climate change. Certainly,
a very good case can be made that damages due to climate change are likely already
well over $10bn per year.
Hence, the increased ocean temperatures and the increased water vapour in the
atmosphere have led to changes in extremes, which have huge impacts on society
and on ecosystems and the environment. Thus, climate extremes exacerbated by
human-induced climate change already pose a serious risk of harm to people’s lives,
 Journal of Energy & Natural Resources Law 19

personal security and property in new ways. The causes of the global warming are clear
and future projections are for more of the same but with increasing magnitude. What are
extreme and unusual events now, boosted by the right kind of circumstances (weather
system), will become commonplace in a decade or two. Without immediate reductions
in fossil fuel emissions, farming may become difficult unless major evolution occurs
(different crops), and by mid-century, many trees and ecosystems will no longer be
viable where they currently stand.
The atmosphere is global; we share these problems with other nations, although
when considering cumulative emissions, the US has been the biggest contributor by
far. As scientists, we can lay out the facts and evidence, and the prospects, but fully
addressing climate change requires government leadership. The costs of the increased
frequency and destructiveness of extreme weather events are not borne by those who
cause the problem. There is still time to manage the problem and avoid the worst
possible outcomes, and there can be major economic advantages as well greater
energy efficiency when transitioning off fossil fuels. It does not have to cost more if
done in the right way. Swift action to reduce emissions and transition off of fossil
fuels can slow and eventually stop further damage to the climate system and water
cycle. The need is swiftly to decarbonise the US energy system, as an essential step
to protect our children and future generations from the real dangers posed by
human-induced climate change.
This is a global problem. We are all together on this spaceship called Earth. What
the US government does with our national energy system and emissions matters
immensely to our ability to preserve a habitable climate for our posterity.

ORCID
Kevin E Trenberth http://orcid.org/0000-0002-1445-1000