The many possible climates from the
Paris Agreement’s aim of 1.5 °C warming
Sonia I. Seneviratne
*, Joeri Rogelj
, Roland Séférian
, Richard Wartenburger
, Myles R. Allen
, Michelle Cain
Richard J. Millar
, Kristie L. Ebi
, Neville Ellis
, Ove Hoegh-Guldberg
, Antony J. Payne
, Carl-Friedrich Schleussner
& Rachel F. Warren
The United Nations’ Paris Agreement includes the aim of pursuing efforts to limit global warming to only 1.5 °C above
pre-industrial levels. However, it is not clear what the resulting climate would look like across the globe and over time.
Here we show that trajectories towards a ‘1.5 °C warmer world’ may result in vastly different outcomes at regional scales,
owing to variations in the pace and location of climate change and their interactions with society’s mitigation, adaptation
and vulnerabilities to climate change. Pursuing policies that are considered to be consistent with the 1.5 °C aim will not
completely remove the risk of global temperatures being much higher or of some regional extremes reaching dangerous
levels for ecosystems and societies over the coming decades.
ince 2010, international climate policy under the United Nations
moved the public discourse from a focus on atmospheric concen-
trations of greenhouse gases to a focus on distinct global temper-
ature targets above the pre-industrial period
. In 2015, this led to the
inclusion of a long-term temperature goal in the Paris Agreement
makes reference to two levels of global mean temperature increase: 1.5 °C
and 2 °C. The former is set as an ideal aim (“pursuing efforts to limit the
temperature increase to 1.5 °C”) and the latter is set as an upper bound
(“well below 2 °C”)
. This change in emphasis allows a better link between
mitigation targets and the required level of adaptation ambition
Assessing the effects of the reduction of anthropogenic forcing through
a single qualifier—namely, global mean temperature change compared
with the pre-industrial climate—however, also entails risks. This decep-
tively simple characterization may lead to an oversimplified perception of
human-induced climate change and of the potential pathways to limit the
impacts of greenhouse gas forcing. We highlight here the multiple ways in
which a 1.5 °C global warming may be realized. These alternative ‘1.5 °C
warmer worlds’ are related to (a) the temporal and regional dimension of
1.5 °C pathways, (b) model-based spread in regional climate responses,
(c) climate noise and (d) a range of possible options for mitigation and
adaptation. We also highlight potential high-risk temperature outcomes
of mitigation pathways currently considered to be consistent with 1.5 °C
owing to uncertainties in relating greenhouse gas emissions to subsequent
global warming, and to uncertainties in relating global warming to asso-
ciated regional climate changes.
Definition of a ‘1.5 °C warming’
Global mean temperature is a construct: it is the globally averaged
temperature of Earth that can be derived from point-scale ground
observations or computed in climate models. Global mean tempera-
ture is defined over a given time frame (for example, averaged over a
month, a year or multiple decades). As a result of climate variability,
which is due to internal variations of the climate system and temporary
naturally induced forcings (for example, from volcanic eruptions), a
climate-based global mean temperature typically needs to be defined
over several decades (at least 30 years according to the World
. Hence, to determine a 1.5 °C global tem-
perature warming, one needs to agree on a reference period (assumed
here to be 1850–1900 inclusive, unless otherwise indicated), and on
a time frame over which a 1.5 °C mean global warming is observed
(assumed here to be of the order of one to several decades). Comparisons
of global mean temperatures from models and observations
are also not straightforward: not all points over Earth’s surface are con-
tinuously observed, leading to methodological choices about how to
deal with data gaps
and with the mixture of air temperature over land
and surface water temperatures of oceans
when comparing full-field
climate models with observational products.
Temporal and spatial dimensions
There are two important temporal dimensions of 1.5 °C warmer worlds:
(a) the time period over which the 1.5 °C warmer climate is assessed;
and (b) the pathway followed before reaching this temperature level, in
particular whether global mean temperature returns to the 1.5 °C level
after previously exceeding it for some time (also referred to as ‘over-
shooting’; see Fig. 1a). As highlighted hereafter, for some components
of the coupled human–Earth system, there are substantial differences
in risk between 1.5 °C of warming in the year 2040, 1.5 °C of warming
in 2100 either with or without earlier overshooting, and 1.5 °C warming
after several millennia at this warming level.
The time period over which 1.5 °C warming is reached is relevant
because some slow-varying elements of the climate system respond with
a delay to radiative forcing and to the associated temperature anomalies.
Hence the status of such slow-varying elements will change over time,
even if the warming is stabilized at 1.5 °C over several decades, centuries
or millennia. This is the case with the melting of glaciers, ice caps and
ice sheets and their contribution to future sea level rise, as well as the
warming and expansion of the oceans, so that a substantial compo-
nent of contemporary sea-level rise is a response to past warming. In
Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland.
International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria.
Environmental Change Institute,
School of Geography and the Environment, University of Oxford, Oxford, UK.
Grantham Institute, Imperial College London, London, UK.
Centre National de Recherches Météorologiques,
Météo-France/CNRS, Toulouse, France.
Department of Global Health, University of Washington, Seattle, WA, USA.
School of Agriculture and Environment, University of Western Australia,
Perth, Western Australia, Australia.
Global Change Institute, University of Queensland, Brisbane, Queensland, Australia.
University of Bristol, Bristol, UK.
Climate Analytics, Berlin, Germany.
IRITHESys, Humboldt University, Berlin, Germany.
Potsdam Institute for Climate Impact Research, Potsdam, Germany.
Tyndall Centre for Climate Change, School of Environmental Sciences,
University of East Anglia, Norwich, UK. *e-mail: firstname.lastname@example.org
7 JUNE 2018 | VOL 558 | NATURE | 41
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