Climate Change: 2013 IPCC Update

5. What changes are projected in the climate system in the future?

• 5.1 Which models and scenarios were used in this climate change assessment?
• 5.2 How are CO2 emissions and the carbon cycle projected to evolve in the future?
• 5.3 How is the climate projected to change in the future?
• 5.4 Is "climate stabilization" possible?
• 5.5 How would extreme events be affected by climate change?

5.1 Which models and scenarios were used in this climate change assessment?

Projections of changes in the climate system are made using a range of climate models that simulate changes based on a set of scenarios of anthropogenic forcings. A new set of scenarios, the Representative Concentration Pathways (RCPs), was used for the climate model simulations carried out for this assessment, these scenarios typically include economic, demographic, energy and simple climate components.

Over the last years, climate models have been continually improved, and there is a very high confidence now that these models reproduce the mean surface temperature patterns on a global scale. There has been also significant improvement in the modelling on the regional scale, but confidence in these models is lower than for the large scale.

Future emissions of greenhouse gases (GHG), aerosol particles and other forcing agents such as land use change are dependent on socio-economic factors, and may be affected by global geopolitical agreements to control those emissions to achieve mitigation. The scenarios used in this assessment that explore what those emissions might be have different targets in terms in radiative forcing by 2100, which range from a ‘strong mitigation’ scenario to a scenario of continuing growth in emissions. More...

5.2 How are CO2 emissions and the carbon cycle projected to evolve in the future?

Between 2012 and 2100, depending on the scenario, Earth System Models (ESM) results imply cumulative fossil fuel emissions ranging between 270 and 1685 billion tonnes of carbon.

The carbon cycle response to future climate and CO₂ changes can be viewed as two strong and opposing feedbacks. One the one hand, there is high confidence that increased atmospheric CO₂ will lead to increased land and ocean carbon uptake and, on the other hand, there is medium confidence that future climate change will decrease land and ocean carbon uptake compared to a constant climate.

This is further supported by paleoclimate observations and modelling indicating that there is a positive feedback between climate and the carbon cycle on century to millennia time scales.

There is high confidence that, at the global scale, the nitrogen cycle , now taken into consideration in the models, will reduce the strength of the carbon feedbacks in the case of land ecosystems.

Changes in the nitrogen cycle, in addition to interactions with CO₂ sources and sinks, affect emissions of nitrous oxide (N₂O) both on land and from the ocean. The human-caused creation of reactive nitrogen has increased steadily over the last two decades and is dominated by the production of ammonia for fertilizer and industry, with important contributions from vegetables production and combustion of fossil fuels. More...

5.3 How is the climate projected to change in the future?

Verification of projections is arguably the most convincing way of establishing the credibility of climate change science. Results of projected changes in carbon dioxide (CO₂), global mean surface temperature and global mean sea level from previous IPCC assessment reports are quantitatively compared with the best available observational estimates. In all cases the observations lie within the range of the projected changes. These anticipated climate changes can be further projected over the next few decades, and even further to the end of the 21^st century. More...

5.3.1 Global mean temperature: In the absence of major volcanic eruptions—which would cause significant but temporary cooling—and, assuming no significant future long term changes in solar irradiance, it is likely that the global mean surface temperature will be higher by 0.3°C to 0.7°C, during the period 2016–2035 compared to 1986–2005 (medium confidence).

Ocean temperatures will very likely continue to increase in the near future. In the short term, it is likely that the frequency and intensity of heavy precipitation events will increase over land. These changes are primarily due to increases in atmospheric water vapor content, but also affected by changes in atmospheric circulation.

Water salinity is likely to increase in the tropical and (especially) subtropical Atlantic, and decrease in the western tropical Pacific over the next few decades. Overall, it is likely that there will be some decline in the Atlantic Meridional Overturning Circulation by 2050 (medium confidence). However, the rate and magnitude of weakening is very uncertain and decades when this circulation increases are also to be expected.

A nearly ice-free Arctic Ocean (sea ice extent less than 1000000 km²) in September is likely before mid-century under the highest emission scenario, with medium confidence. It is very likely that there will be further shrinking and thinning of Arctic sea ice cover, and decreases of northern high-latitude spring time snow cover and near surface permafrost as global mean surface temperature rises. More...

5.3.2

• Global mean temperatures will continue to rise over the 21^st century under all of the scenarios. From around the mid-21^st century, the rate of global warming begins to be more strongly dependent on the scenario: the likely global-mean surface temperatures increases are projected as 0.3 to 4.8°C
• For the period 2081–2100, global temperatures are projected to likely exceed 1.5°C above preindustrial for three scenarios (high confidence) and are likely to exceed 2°C above preindustrial for two (high confidence).
• Warming above 4°C by 2081–2100 is unlikely in all RCPs (high confidence) except for the scenario of continuing growth of emissions, where it is as likely as not (medium confidence).
• In the emission scenario with the heaviest mitigation, temperature increases above 2°C relative to preindustrial is unlikely (medium confidence).

it is virtually certain that, in most places, there will be more hot and fewer cold temperature extremes as global mean temperatures increase. Increases in the frequency, duration and magnitude of hot extremes along with heat stress are expected, however occasional cold winter extremes will continue to occur.

It is virtually certain that, in the long term, global precipitation will increase with increased global mean surface temperature. Some regions will experience increases, other regions will experience decreases, and yet others will not experience significant changes at all.

It is very likely that the Arctic sea ice cover will continue to shrink and get thinner year-round in the course of the 21^st century. Projections give average reductions in Arctic sea ice over the 21^st century of ranging from 8% to 34% in February and from 43% to 94% in September, with a nearly ice-free Arctic Ocean in September likely before the middle of the century for the highest emission scenario (medium confidence) (Figure TS.17).

In the Antarctic, a decrease in sea ice extent and volume is expected, but with low confidence. Projected decreases in sea ice extent range from 16% to 67% in February and from 8% to 30% in September for 2081‒2100 compared to 1986‒2005.

The available evidence indicates that global warming greater than a certain threshold (generally estimated to be between 2°C and 4°C of warming, with some studies suggesting that it could as low as 1°C) would lead to the near- complete melting of the Greenland Ice Sheet over a millennium or more, causing a global mean sea level rise of about 7 m but a likely range cannot be quantified.

Northern Hemisphere snow cover and the extent of permafrost are both projected to decrease as global temperatures rise over the coming century. However, the existing studies of the carbon balance of permafrost do not yield consistent results, beyond the fact that present-day permafrost will become a net emitter of carbon during the 21^st (low confidence)).

Over the course of the 21^st century, the global ocean will warm in all scenarios. In some regions by the end of the century, ocean warming is projected to exceed 0.5°C to 2.5°C in the top few hundred meters and 0.3°C to 0.7°C at a depth of about 1 km. Due to the long time it takes for energy to transfer from the surface to deeper water, ocean warming will continue for centuries, and will result in a continued contribution to sea level.

The Global mean sea level is projected to rise 0.26 to to 0.81 m before the end of the 21^st century, with the thermal expansion of water being the main driver on this rise.. it is virtually certain that sea level rise will continue beyond 2100, and go on for centuries to millennia. The few available model results that go beyond 2100 indicate global mean sea level rise above the pre-industrial level by 2300 to be between 1 m to more than 3 m (medium confidence) depending on the scenario.

With very high confidence, uptake by oceans of anthropogenic CO₂ will continue under all four scenarios through to 2100, and this will bring further acidification of the ocean. Oxygen levels are projected to decrease with warming. The future evolution of the CO₂ uptake by land is much more uncertain.

There is high confidence that climate change will partially offset increases in global land and ocean carbon sinks caused by rising atmospheric CO₂. Tropical ecosystems will store less carbon in a warmer climate. At high latitudes, warming will increase land carbon storage, but models do not take into account the release of carbon from permafrost. More...

5.3.3

• Dates of the beginning of Monsoon season are likely to become earlier, or to not change much over the 21^st century, while the dates of the end of the seasons are very likely to delay, resulting in a lengthening of the monsoon season.
• For tropical rainfall, it is virtually certain that precipitation change will vary from place to place, increasing in some regions and decreasing in some others. Rainfall is likely to increase near the currently rainy regions, and to also increase in areas where ocean warming is greater.
• The El Niño-Southern Oscillation will very likely remain a dominant driver of year-to -year variability. The variability in rainfall between regions that it brings is likely to intensify. (low confidence)
• For tropical cyclones, projections for the 21^st century indicate that it is likely that their global frequency will either decrease or remain essentially unchanged, and at the same time their intensity will likely increase.
• The North Atlantic Oscillation, Southern Annular Mode and Atlantic Multi-Decadal Oscillation all are likely to be affected by climate changes. But it is not clear what implications those changes will have.

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5.4 Is "climate stabilization" possible?

Climate stabilization can mean:

• to stabilize of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system, which is the ultimate objective of the United Nations Framework Convention on Climate Change (UNFCCC) ;
• to limit the global temperature increase, which has been the focus of recent policy discussions. The most widely discussed being 2°C above pre industrial levels;
• to return the level of atmospheric CO₂ below 350 ppm.

Stabilization of the global temperature does not imply stabilization for all aspects of the climate system. Processes related to vegetation, ice sheets, deep ocean warming and associated sea level rise as well as potential feedbacks have their own intrinsic long timescales. Ocean acidification will very likely continue in the future as long as the oceans continue to take up atmospheric CO₂. Committed land ecosystem carbon cycle changes will manifest themselves further beyond the end of the 21^st century. Climate impacts are diverse, some might be beneficial, and there is no objective measure of when a dangerous threshold is reached. A discussion on the merits of specific targets is beyond the scope of this assessment.

One approach to reach climate stabilization is geo-engineering, defined as the deliberate large-scale intervention in the Earth system to counter undesirable impacts of climate change on the planet:

• CO₂ emission reduction through the setup of artificial carbon sinks could be an option, although the large scale needed to reduce CO₂ in any significant way is a serious limitation;
• Another approach is Solar Radiation Management, where the injection of aerosols in the atmosphere would reduce the radiative forcing;
• Attempts to affect cloud cover and brightness are also foreseen, but these do not address the other problems cause by CO₂ increase such as ocean acidification.

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5.5 How would extreme events be affected by climate change?

Assessing changes in climate extreme events poses unique challenges, not just because of the rare nature of these events, but because they invariably happen in conjunction with disruptive conditions. They are strongly influenced by both small and large-scale weather patterns, modes of variability, thermodynamic processes, land-atmosphere feedbacks and antecedent conditions. However, much progress has been made since the 2007 assessment.

For the near- and long-term, scenario’s projections confirm a clear tendency for increases in heavy precipitation events, although with large regional variations. There is new evidence that the influence of anthropogenic forcing may be detected separately from the influence of natural forcing at global scales and in some continental and sub-continental regions.

For extreme events such as floods, droughts, and cyclones there are still a lot of uncertainties when it comes to establishing a trend of change or establishing projections. Under the highest greenhouse gases emission scenarios, which would bring about the largest changes in temperatures and precipitations, projections indicate an increased risk of drought over the course of the 21^st century in regions that are already dry.

Sea level: It is likely that the magnitude of extreme high sea level events has increased since 1970 and that most of this rise can be explained by increases in mean sea level. With sea level projected to continue rising over the course of the 21^st century, it is very likely that there will be further increase in sea level extremes.

Droughts: An updated assessment of the observational evidence indicates that the previous report AR4 conclusions regarding global increasing trends in hydrological droughts since the 1970s are no longer supported. There is now low confidence in the attribution of changes in drought over global land since the mid-20^th century to human influence More...