The Paris Agreement advocates that humanity should consider limiting global warming to no more than 1.5 degrees Celsius (°C) above pre-industrial temperatures, well below the previously discussed threshold of 2 °C. The announcement sparked a surge of research to understand the practicality and implications of the lower limit. Here, Philip Kraaijenbrink and colleagues simulate the effect of warming on the glaciers in the high mountains of Asia and show that, in a world that warms by just 1.5 °C, about 65 per cent of glacier mass will remain by 2100. But keeping warming below the 1.5 °C threshold is an ambitious goal. At the other extreme, scenarios that include continued high rates of greenhouse gas production instead suggest that only about 35 per cent of mass will remain by 2100.
Targeting 1.5 °C
In December 2015, representatives from 195 nations met in Paris to negotiate an international agreement to combat climate change. The resulting ‘Paris Agreement’ codified an aspiration to limit the level of global temperature rise to 1.5 °C above pre-industrial levels — lower than the previously generally agreed target of 2 °C. From a research standpoint, a more ambitious temperature target poses many questions that could draw scientific and intellectual attention and resources. Furthermore, the timescales in which researchers must decide how to engage with this new policy context is very short.
The Intergovernmental Panel on Climate Change has agreed to publish a special report on the costs and implications of the 1.5 °C target in 2018. In order to inform that process, researchers must decide which efforts to prioritise and begin work almost immediately. But deciding what can and should be delivered is far from trivial. This evolving collection draws together content from Nature Climate Change, Nature Geoscience, Nature Communications, Nature Energy and Nature to provide comment on how research might best inform decisions about limiting climate warming as well as presenting pertinent new research that addresses this very question.
Nations are currently pursuing efforts to constrain anthropogenic warming to 1.5 °C. In such a world, model projections suggest the Arctic will be ice-free every one in forty years, compared to one in every five under stabilized 2 °C warming.
If CO2 emissions after 2015 do not exceed 200 GtC, climate warming after 2015 will fall below 0.6 °C in 66% of CMIP5 models, according to an analysis based on combining a simple climate–carbon-cycle model with estimated ranges for key climate system properties.
Carbon release from permafrost thaw would substantially decrease the amount of carbon emissions required to meet climate targets, according to climate simulations.
Rising seas are a legacy of present and future climate change. Here the authors show that under the Paris Agreement, emissions in the next decades have a strong influence on the amount of sea level rise in the centuries to come, with the uncertainty dominated by ice-sheet contributions.
Arctic sea ice cover has declined significantly in recent years. Model simulations suggest the probability of an ice-free Arctic will be 100% under 2 °C, but 30% under 1.5 °C, motivating efforts to constrain anthropogenic warming.
The populous global land monsoon region has been suffering from extreme precipitation. Here, the authors show that limiting global warming to 1.5 °C instead of 2 °C could reduce areal and population exposures to baseline once-in-20-year rainfall extremes by 25% (18–41%) and 36% (22–46%), respectively.
If the world can meet the target of limiting global warming to 1.5 °C, economic damage will probably be greatly reduced, especially in poorer countries.
Energy investment needs for fulfilling the Paris Agreement and achieving the Sustainable Development Goals
The scale and nature of energy investments under diverging technology and policy futures is of great importance to decision makers. Here, a multi-model study projects investment needs under countries’ nationally determined contributions and in pathways consistent with achieving the 2 °C and 1.5 °C targets as well as certain SDGs.
Residual CO2 emissions from fossil fuels limit the likelihood of meeting the goals of the Paris Agreement. A sector-level assessment of residual emissions using an ensemble of IAMs indicates that 640–950 GtCO2 removal will be required to constrain warming to 1.5 °C.
Land-based mitigation for meeting the Paris climate target must consider the carbon cycle impacts of land-use change. Here the authors show that when bioenergy crops replace high carbon content ecosystems, forest-based mitigation could be more effective for CO2 removal than bioenergy crops with carbon capture and storage.
Scenarios that constrain end-of-century radiative forcing to 1.9 W m–2, and thus global mean temperature increases to below 1.5 °C, are explored. Effective scenarios reduce energy use, deploy CO2 removal measures, and shift to non-emitting energy sources.
It is unclear how extreme positive Indian Ocean Dipole will respond to 1.5 °C of warming. Here the authors show that the frequency of these events increases linearly with warming, doubling at 1.5 °C from the pre-industrial level, but plateaus thereafter.
A 1.5 °C temperature target can have varying atmospheric CO2 concentrations associated with it. GCM simulations reveal CO2 increases have a direct impact on climate extremes, highlighting the need for climate policy to complement temperature goals with CO2 targets.
Quantifying the rural residential energy transition in China from 1992 to 2012 through a representative national survey
Use of wood and crop residue for cooking and heating in rural China is a significant source of carbon emissions and air pollution. Using a survey of more than 34,000 households, researchers show that between 1992 and 2012 usage of these fuels decreased by much more than previous estimates, due primarily to rising incomes.
Record-breaking climate extremes in Africa under stabilized 1.5 °C and 2 °C global warming scenarios
The record hot year of 2015 in Africa had devastating impacts. The likelihood of future annual temperature extremes over Africa exceeding those of 2015 are 91% and 100% in 1.5 °C and 2 °C worlds, respectively, stressing the benefits of limiting future anthropogenic warming.
Scenarios that constrain warming to 1.5 °C currently place a large emphasis on CO2 removal. Alternative pathways involving lifestyle change, rapid electrification and reduction of non-CO2 gases could reduce the need for such negative emission technologies.
Limiting warming to 1.5 °C requires staying within an allowable carbon budget. An analysis of warming and carbon budgets from the past decade shows that the median remaining budget is 208 PgC, corresponding to about 20 years of emissions at the 2015 rate.
Glaciers outside Greenland and Antarctica have been rapidly losing mass. Contemporary ice declines are shown to be a response to past greenhouse gas emissions, with present mitigation efforts unlikely to be beneficial in preventing future short-term ice loss.
The Paris Agreement zero-emissions goal is not always consistent with the 1.5 °C and 2 °C temperature targets
While well-known for its temperature targets, the Paris Agreement also aims for net zero GHG emissions. IAM results reveal net zero GHG emissions are not always required to meet the temperature targets, and that net zero CO2 emissions are a more suitable aim.
Arid regions are projected to expand in the future. An ensemble of climate model simulations reveals that limiting anthropogenic warming to 1.5 °C instead of 2 °C can markedly reduce the area undergoing, and thus the population exposed to, aridification.
A reasonable interpretation of the Paris Agreement may well still be technically achievable without the need for net negative emissions or excessively stringent policies according to climate–carbon-cycle modelling.
A combination of the level and rate of human-induced warming allows estimation of remaining emission budgets to peak warming across a broad range of scenarios, suggests an analysis of emissions budgets expressed in terms of CO2-forcing-equivalent emissions.
Climate feedbacks associated with wetland methane emissions and permafrost-thaw carbon release substantially reduce available carbon budgets to achieve temperature targets, suggest simulations with a climate–land-surface model system.
A 1.5 °C climate target implies total emissions of carbon from the start of 2017 must fall below 195 to 205 PgC, according to an observationally constrained very large ensemble of simulations with an efficient Earth system model.
This paper presents interrelated indicators for tracking progress towards the Paris Agreement. Findings show broad consistency with keeping warming below 2 °C, but technological advances are needed to achieve net-zero emissions.
Permafrost loss can be projected by considering its distribution against warming air temperatures. Using observations to constrain loss estimates, this study investigates loss under different levels of warming.
COP21 led to a global commitment to decarbonization before 2100 to combat climate change, but leaves the timing and scale of mitigation efforts to individual countries. Here, the authors show that global carbon emissions need to peak within a decade to maintain realistic pathways for achieving the Paris Agreement.
Climate change is expected to alter ocean ecology, and to potentially impact the ecosystem services provided to humankind. Here, the authors address how rapidly multiple factors that affect marine ecosystems are likely to develop in the future ocean and the remedial effects climate mitigation might have.
Re-assessment of net energy production and greenhouse gas emissions avoidance after 40 years of photovoltaics development
While the photovoltaic industry aims to achieve cleaner energy production, it consumes energy and emits greenhouse gases during production and deployment. Here, Louwenet al. show that the industry has likely already reached break-even points for both greenhouse gases emissions and electricity consumption.
Optimal bioenergy power generation for climate change mitigation with or without carbon sequestration
Prior mitigation assessments of atmospheric CO2 removal rely on bioenergy carbon capture and storage (BECCS), excluding bioenergy-biochar systems (BEBCS). Here, Woolf et al. find that BEBCS offers an alternative cost-effective solution, and may allow earlier CO2removal at a lower carbon price.
Research and debate are intensifying on complementing CO2 emissions reductions with hypothetical climate geoengineering techniques. Here, the authors assess their potentials, uncertainties and risks, and show that they cannot yet be relied on to significantly contribute to meeting the Paris Agreement temperature goals.
Action needs to be taken to limit the impacts of climate change, however, human rights and the right to development need to be preserved. This Perspective weighs the risks of action and inaction on achieving a just transition to a low-carbon world.
The results of efforts to limit global mean warming to below 1.5 °C may include many possible future world climates.
A review of Earth system changes associated with past warmer climates provides constraints on the environmental changes that could occur under warming of 2 °C or more over pre-industrial temperatures.
Land management with the aim of reducing incoming solar radiation could help with regional-scale climate adaptation and mitigation as well as ecosystem services, and avoids several shortcomings of global geoengineering.
Scenario analyses suggest that negative emissions technologies (NETs) are necessary to limit dangerous warming. Here the authors assess the biophysical limits to, and economic costs of, the widespread application of NETs.
This Perspective considers the potential mitigation contribution of carbon capture and utilization, such as chemical conversation or to enhance oil recovery. The authors find it will account for a small amount of the required total mitigation effort.
There are discernible differences in climate impacts between 1.5 °C and 2 °C of warming. The extent of countries' near-term mitigation ambition will determine the success of the Paris Agreement's temperature goal.
The principal climate goal of the Paris Agreement of December 2015 is to hold the increase in the global average temperature to well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 degrees above pre-industrial levels. This Perspective assesses the national plans submitted to the Paris meeting for post-2020 action to reduce global greenhouse gas emission by 2030. It also provides projections for global mean temperature increase over the twenty-first century that would be consistent with the present national plans and discusses options that may help to reduce greenhouse gas emissions to levels that are more consistent with maintaining a reasonable chance of meeting the well below 2 degrees Celsius climate target.
A long-term goal for climate policy can only be agreed through political processes, but science can inform these through mapping policy choices and the risks they create. Recommendations for the practical use of the IPCC's Fifth Assessment Report are provided.
Opinion & Comment
The benefits of limiting global warming to the lower Paris Agreement target of 1.5 °C are substantial with respect to population exposure to heat, and should impel countries to strive towards greater emissions reductions.
Most scenarios to meet the Paris Agreement require negative emissions technologies. The EU has assumed a global leadership role in mitigation action and low-carbon energy technology development and deployment, but carbon dioxide removal presents a serious challenge to its low-carbon policy paradigm and experience.
Although nearly all 2 °C scenarios use negative CO2 emission technologies, only relatively small investments are being made in them, and concerns are being raised regarding their large-scale use. If no explicit policy decisions are taken soon, however, their use will simply be forced on us to meet the Paris climate targets.
Nation states need to incentivize negative emissions technologies if they are to take the decarbonization of whole energy systems seriously. This incentivization must account for public values and interests in relation to which technologies to incentivize, how they should be incentivized and how they should be governed once incentivized.
Temperature overshoot scenarios that make the 1.5 °C climate target feasible could turn into sources of political flexibility. Climate scientists must provide clear constraints on overshoot magnitude, duration and timing, to ensure accountability.
Upward estimates for carbon budgets are unlikely to lead to action-focused climate policy. Climate researchers need to understand processes and incentives in policymaking and politics to communicate effectively.
The remaining carbon budget consistent with limiting warming to 1.5 °C allows 20 more years of current emissions according to one study, but is already exhausted according to another. Both are defensible. We need to move on from a unique carbon budget, and face the nuances.
The Intergovernmental Panel on Climate Change is preparing a report on keeping global warming below 1.5 °C. How the panel chooses to deal with the option of solar geoengineering will test the integrity of scientific climate policy advice.
The Paris Agreement introduced three mitigation targets. In the future, the main focus should not be on temperature targets such as 2 or 1.5 °C, but on the target with the greatest potential to effectively guide policy: net zero emissions.
Stéphane Hallegatte, Katharine J. Mach and colleagues urge researchers to gear their studies, and the way they present their results, to the needs of policymakers.
The Paris Agreement duly reflects the latest scientific understanding of systemic global warming risks. Limiting the anthropogenic temperature anomaly to 1.5–2 °C is possible, yet requires transformational change across the board of modernity.
An IPCC Special Report on 1.5 °C should focus on resolving fundamental scientific and political uncertainties, not fixate on developing unachievable mitigation pathways.
The academic community could make rapid progress on quantifying the impacts of limiting global warming to 1.5 °C, but a refocusing of research priorities is needed in order to provide reliable advice.
The adoption of the Paris Agreement is a historic milestone for the global response to the threat of climate change. Scientists are now being challenged to investigate a 1.5 °C world — which will require an accelerated effort from the geoscience community.