This is the un-condensed version of a Nature commentary on the need to move past the widely-used 2-degree (C) threshold for global warming danger -- written by David Victor and Charles Kennel of the University of California, San Diego. For more, read the related Dot Earth post at http://dotearth.blogs.nytimes.com

1. 1
Beyond 2 degrees: Setting New Goals for Global Warming Diplomacy
8 September 2014
Charles F. Kennel and David G. Victor
For nearly a decade, international diplomacy has focused on stopping global warming at
2 degrees above pre-industrial levels. This goal—bold yet easy to comprehend—has
been accepted uncritically and has proved to be politically powerful. Nearly every grand
plan to stop climate change is compared with this goal.1, 2, 3, 4 Political systems—from
California to the whole of the European Union (EU)—have organized climate policy
organized around 2 degrees.5, 6 A vast number of policy analyses, including much of the
new Intergovernmental Panel on Climate Change (IPCC) report on emissions mitigation,
have examined exactly how warming can be stopped at 2 degrees.7,8
Bold simplicity must now face reality. On both political and scientific fronts, the 2
degree goal is proving wrong-headed. Politically, the goal bears no relationship to what
real governments can actually implement globally. It has allowed governments to
pretend they are taking serious actions when, in reality, almost nothing has been achieved
to slow global warming. Scientifically, the facts are even more troubling. The last 16
year “pause” in the growth of average global surface temperature underscores that there
are better ways to measure the real stress that humans are placing on the climate system
and the risks they expose themselves to.
New goals are needed. Better approaches that would set goals in terms of what humans
actually emit—rather than global temperatures, which poorly coupled to what humans
can actually control through policy. New goals should also work with an array of
planetary vital signs—such as changes in the atmospheric radiation balance and ocean
heat content—that are better rooted in the real, scientific understanding of climate drivers
and risks.
The Politics of Goals
From the very beginning of climate policy efforts, goals have proved important yet
difficult to articulate.
With the UN Framework Convention on Climate Change (UNFCCC) diplomats set the
goal of preventing “dangerous anthropogenic interference in the climate system.”9 In the
aftermath of the UNFCCC there were many requests to clarify the meaning of Article 2 of
the Convention.10 But those efforts never bore fruit because Article 2, by design, sets

2. goals that co-mingle questions of risk with those of socioeconomic priorities such as
economic growth, and poverty alleviation. Despite many conferences and volumes
published on the matter, the real meaning of Article 2 remains no clearer today than it did
back in 1992 when it was drafted.
The 2009 UNFCCC Conference of Parties meeting in Cancun addressed this problem by
reframing the goal in terms of global temperature.11 There wasn’t much scientific basis
for that effort—or for the 2 degree number they adopted—but this wishful construction
allowed diplomats to swim with the tide. The Copenhagen Conference the year before
had already loosely adopted 2 degrees as a goal for global climate diplomacy—and cited,
loosely, the IPCC for scientific support even though the IPCC offered no such clear policy
advice.12 The EU had already adopted the 2 degree goal in 1996, and the Group of 8 (G8)
industrial countries along with other international forums had steadily over the 2000s been
adopting increasingly precise formulations of the same 2 degree goal.13, 14, 15
Politically, the troubles for the 2 degree goal arise on two fronts. First, there is growing
evidence that the goal itself is unachievable.16, 17 To be sure, there are model runs that
show it is still possible, just, to make deep planet-wide cuts in emissions that would be
consistent with stopping warming at 2 degrees by 2100.18 But those runs are based on
heroic assumptions—such as almost immediate global cooperation and widespread
availability of technologies such as bioenergy carbon capture and storage (BECCS) that
don’t exist even in scale demonstration and are far from ever being commercially viable.19
Second, the political usefulness of goals requires not just that they be bold and
inspirational but also that real governments be able to plan around them. Yes, goals must
be visible to inspire political action, but they must also be within reach of what
governments can actually implement.20, 21 It is this connection to practicality that is
missing from the 2 degree goal. A better model, for example, are the eight bold
Millennium Development Goals (MDGs) adopted by the United Nations in 2000. By
themselves the MDGs did not inspire the practical actions needed to achieve them. The 8
goals had to be turned into 21 targets and 60 detailed indicators—measurable, practical,
and connected to what governments, NGOs, aid organizations and others could actually
deliver.22 Climate scientists and policy makers—should do something similar for climate
goals.
A Troubling Pause
The climate system has already demonstrated that global temperatures bear little direct
relationship to the accumulating stresses on the climate system. Since 1998 the planet
has seen a “hiatus” in measured warming, following with one of the largest sea surface El
Nino events in decades that year. The global temperature has remained elevated but
relatively constant since then. Although global temperature growth has paused, other
indicators point to rapid change in the climate system. Two successive assessments of
found the Arctic region warming rapidly23, 24 ; polar warming, was considerably faster in
2011 than seven years earlier.25 Declines in albedo (reflectivity) with melting snow and
2

3. sea ice are accelerating Arctic warming.26, 27 ; a recent study suggests that this albedo
feedback may be stronger than previous models had assumed.28
There was no hiatus in weather disasters either.29 A growing number of studies has
linked extreme weather events to global climate change even though global temperatures,
themselves, have not changed on average.30, 31 Because extreme weather events are so
visible, they are an important component of the public debate about climate change. And
serious studies on climate impacts show that extreme events will dominate planning for
climate adaptation.32
Satellite measurements of the balance of incoming and outgoing radiant energy at the top
of the atmosphere make it clear that the climate system was absorbing energy during the
hiatus.33 If that energy did not go into the atmosphere and earth’s surface, it must have
gone into the oceans. Only in the past two years has it become clear what has been
happening. There seem to be two related explanations. The Pacific Ocean has been in a
prolonged cool counter-cycle to the El Nino—a La Nina-like phase. The sea surface
temperature cools by as much as two degrees as a La Nina progresses, enough when
factored into computations of the global surface temperature to depress it. In a La Nina,
equatorial trade winds push the tropical surface waters westward. These waters are
heated by exposure to the tropical sun; when they arrive at the so-called Pacific Warm
Pool near Indonesia, they have no place to go, so they plunge to depth, carrying with
them the heat they acquired on the trip across the Pacific. The westward trade winds
have indeed been unusually strong since the hiatus began34. Climate models driven by the
observed La Nina effect reproduce the global temperature time history.35 The North
Atlantic and Southern Oceans also began sequestering heat energy when the hiatus
began36. The measured global Ocean Heat Content has indeed increased at depth, while
the surface ocean heat content remained constant.37
Just as policy makers set surface temperature as the single most prominent metric for
policy success, the planet began demonstrating that it is a poor indicator of actual stress.
3
Vital Signs for the Planet
A single index of climate change risk—a crisp answer to the request posed in Article 2 of
the UNFCCC—would be wonderful. Such an index, however, does not exist. Instead, a
basket of indicators is needed to measure the varied stresses that humans are placing on
the climate system and their possible impacts. Doctors call their basket of health indices
vital signs. We think the same approach is needed for climate.
The best indicator has been there all along—the concentrations of CO2 and the other
greenhouse gases, or equivalently, the change in radiative forcing associated with those
concentrations. Charles David Keeling’s record of CO2 accumulation, now continued by
other hands, is just about the only fixed point in the entire climate debate.38 Reliable
measurements of other anthropogenic greenhouse gases are regularly made and attributed

4. to their sources39. For some, such as methane or soot, there remain important uncertainties
about the link between human emissions and measured concentrations.40 But policy
efforts to improve measurement and control of those warming agents—such as through the
Climate and Clean Air Coalition—are advancing as well. The best way to measure policy
progress is to set goals in concentrations and watts per square meter of the gases and
aerosols that cause climate change.
Policy makers should also make use of two other indicators that, unlike globally averaged
temperature, are more fundamental and reliable measures of the actual human stress on the
climate: the ocean heat content (OHC) and high latitude temperature. Unlike globally
averaged temperature, OHC is a better measure of the full heat impact that greenhouse
gases exert on the climate system. OHC has a key role because the oceans take up 93% of
the energy added by anthropogenic greenhouse warming41. Since energy stored in the deep
oceans will be released over decades to centuries, OHC measures the committed long-term
risk to future generations and planetary-scale ecology. And high latitude temperatures,
unlike global temperature, are fundamentally more sensitive to changes in the planetary
heat balance—they are like canaries in the coal mine. High latitude temperatures, which
have warmed at twice the global average, drive changes in weather and climate through
the rest of the climate system.42
With additional research and instrumentation other important indicators could be
developed as well—so that, in time, climate goals can reflect a basket of diverse
indicators. Better measures for the trends in the difference between total solar radiation
energy in and long-wavelength infrared radiation out at the top of the atmosphere indicate
how much energy the total climate system is taking up or releasing.
Policy makers should also urge scientists to develop better measures of short-term risk to
society and infrastructure. Higher sea levels, stronger storm surges, prolonged droughts
and other extreme events will create risks to vulnerable populations, infrastructure and
ecosystems. On this front, the science is improving quickly. For example, IPCC AR5 has
reported that the sea level rise budget has been nearly closed for the first time—that is, the
observed rate can be roughly accounted for by summing over the various sources.43
Satellite altimetry can differentiate local sea level rise rates, from the global rate, and,
combined with digital elevation maps and local subsidence rates, provide estimates of the
populations and economic assets at risk.44 Given generally accepted assessment standards,
local estimates could be added up globally.
What’s ultimately needed is a volatility index that measures the evolving risk from
extreme events—so that planetary, average indicators such as global greenhouse gas
concentrations can be coupled to local information on what people ultimately care most
about. One place to start is an index that measures the total area during the year in which
extreme conditions that depart by three standard deviations from the local and seasonal
mean occur.45 Looking to the future, the short-term variations in TOA radiation imbalance
could conceivably help predict how such an extreme event index will behave in the mid-term.
Better extreme event indexes would help to underscore for policy makers that the
4

5. risks in climate are not from the averages but from the variations.46 It would set a more
realistic framework for adaptation planning.
In the end, the public needs to know what it is being asked to pay for. On that front, “CO2
concentration” or “ocean heat content” are not nearly effective as “temperature” in
conveying to the person in the street a sense of what is at risk. The case for bold and
simple measures has always been the refuge for those who advocate 2 degrees as a goal.
Yet complexity in setting goals is, in fact, quite familiar to the public. Doctors cannot
predict how their patients’ lives will change when a single vital sign changes; patients
have come to understand the importance of a basket of vital signs and a focus on risk
management. When it comes to the planet, experts and laypeople alike must do the same.
The window of opportunity for better thinking is now open. This fall a big push on
climate policy begins—with the aim of crafting a new treaty by late 2015 at a major
diplomatic meeting in Paris. Getting serious about climate change requires wrangling not
just about the cost of emission goals, sharing the burdens and crafting new international
funding mechanisms—the topics that, already, are dominating the agenda. Diplomats must
also accept that the 2 degree goal is not well-designed, and we in the expert community
must help them understand what’s wrong with that goal and how better goals might be
crafted.
The scientific community should be asked to organize its voice on better indicators. A
constructive step would be an international conference that examines what would be
necessary to ready today’ research measurements to become tomorrow’s policy indicators.
5
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