Long-term Climate Change:PROJECTIONS, COMMITMENTS AND IRREVERSIBILITY

Abdalah MOKSSIT –

AR5 IPCC OUTREACH RABAT 04/05/2015

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The Coupled Model Intercomparison Project Phase 5 (CMIP5) presents an unprecedented level of information on which to base projections including new Earth System Models with a more complete representation of forcings, new Representative Concentration Pathways (RCP) scenarios and more output available for analysis.

New experiments and studies have continued to work towards a more complete and rigorous characterization of the uncertainties in long-term projections, but the magnitude of the uncertainties has not changed significantly since AR4.

Pour établir le cinquième Rapport d’évaluation du GIEC, la communauté scientifique a défini un ensemble de quatre nouveaux scénarios, appelés profils représentatifs d’évolution de concentration (RCP).

Ces RCP sont identifiés par leur forçage radiatif total approximatif pour l’année 2100 par rapport à 1750:

-2,6 W m-2 pour le RCP2,6 scénario d’atténuation conduisant à un niveau de forçage très bas

- 4,5 W m-2 pour le RCP4,5 scénario de stabilisation- 6,0 W m-2 pour le RCP6,0 scénario de stabilisation- 8,5 W m-2 pour le RCP8,5 scénario aux émissions GES très élevées

Les RCP peuvent ainsi représenter toute une gamme de politiques climatiques pourle XXIe siècle, par comparaison avec le rapport spécial sur les scénarios d’émissions (SRES), dont les scénarios d’émissions étaient utilisés dans les troisième et quatrième Rapports d’évaluation, et qui ne prévoyait pas de politique climatique.

Global mean temperatures will continue to rise over the 21st century if greenhouse gas (GHG) emissions continue unabated.

Temperature change will not be regionally uniform. There is very high confidence that globally averaged changes over land will exceed changes over the ocean at the end of the 21st century by a factor that is likely in the range 1.4 to 1.7.

Annual mean surface air temperature changeMulti-model ensemble average of surface air temperature change (compared to 1986–2005 base period) for 2046–2065, 2081–2100, 2181–2200 for RCP2.6, 4.5, 6.0 and 8.5.

Temperature (left) and precipitation (right) change patterns derived from transient simulations from the CMIP5 ensembles, scaled to 1°C of global mean surface temperature change. The patterns have been calculated by computing 20-year averages at the end of the 21st (top) and 22nd (bottom) centuries and over the period 1986–2005 for the available simulations under all RCPs, taking their difference (percentage difference in the case of precipitation) and normalizing it, grid-point by grid-point, by the corresponding value of global average temperature change for each model and scenario.

Global mean temperatures will continue to rise over the 21st century under all of the RCPs. From around the mid-21st century, the rate of global warming begins to be more strongly dependent on the scenario .

It is virtually certain that, in most places, there will be more hot and fewer cold temperature extremes as global mean temperatures increase.

These changes are expected for events defined as extremes on both daily and seasonal time scales. 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.

Projected Long-term Changes in the Water Cycle

Changes in average precipitation in a warmer world will exhibit substantial spatial variation. Some regions will experience increases, other regions will experience decreases and yet others will not experience significant changes at all. There ishigh confidence that the contrast of annual mean precipitation between dry and wet regions and that the contrast between wet and dry seasons will increase over most of the globe as temperatures increase.

Annual surface evaporation is projected to increase as global temperatures rise over most of the ocean and is projected to change over land following a similar pattern as precipitation.

Prominent areas of projected decreases in evaporation include southern Africa and north western Africa along the Mediterranean. Surface drying in these regions as global Temperatures increase is likely with high confidence by the end of this century under the RCP8.5 scenario

Extreme Events in the Water Cycle

Long-term Projections of Sea Level Change

Long-term Projections of Sea Level Change

It is very likely that the Arctic sea ice cover will continue shrinking and thinning year-round in the course of the 21st century as global mean surface temperature rises.

At the same time, in the Antarctic, a decrease in sea ice extent and volume is expected, but with low confidence.

It is very likely that NH snow cover will reduce as global temperatures

rise over the coming century. A retreat of permafrost extent with rising global temperatures is virtually certain. Fig. 12.29

The global ocean will warm in all RCP scenarios. The strongest ocean warming is projected for the surface in subtropical and tropical regions.

Best estimates of  ocean warming in the top one hundred meters are about 0.6°C (RCP2.6) to 2.0°C (RCP8.5), and about 0.3°C (RCP2.6) to 0.6°C (RCP8.5) at a depth of about 1 km by the end of the 21st century.

Long-term Climate Change, Commitment and Irreversibility

Global temperature equilibrium would be reached only after centuries to millennia if radiative Forcing were stabilized.

A large fraction of climate change is largely irreversible on human time scales, unless net anthropogenic CO were strongly negative over a sustained period.

For scenarios driven by CO2 alone, global average temperature is projected to remain approximately constant for many centuries following a complete cessation of emissions. The positive commitment from CO2 may be enhanced by the effect of an abrupt cessation of aerosol emissions, which will cause warming. By contrast, cessation of emission of shortlived GHGs will contribute a cooling.

Some aspects of climate will continue to change even if temperatures are stabilized. Processes related to vegetation change, changes in the ice sheets, deep ocean warming and associated sea level rise and potential feedbacks linking for example ocean and the ice sheets have their own intrinsic long time scales and may result in significant changes hundreds to thousands of years after global temperature is stabilized.

Thank you for your attention