Climate change, impacts and vulnerability in Europe ... - MemoFin.fr

Changes in the climate systemdamming of rivers and lakes. Some human activitieshave a cooling effect, in particular the emission ofaerosols (i.e. sulphates, smoke, dust and haze) thatreflect part of the incoming sunlight and certainland-use changes that increase the reflectivity of theland surface (see Figure 2.2).2.1.2 Observed climate change and its attributionThe IPCC AR4 (IPCC, 2007) concludes withvery high confidence that human activities havecontributed to the warming of the global climatesince at least 1750. It estimates that the totalwarming effect of human activities is at least10 times larger than that of natural factors, inparticular changes in solar activity. The AR4 furtherconcludes that the warmth since the mid‐20thcentury is exceptional in at least the last 1 300 years,and that the observed rapid increase in globalaverage temperatures since the mid-20th centuryis very likely due to the observed increase in GHGconcentrations due to human activities. In otherwords, humans have now become the dominatingcause of changes in global climate on decadal andcentennial time scales.The observed increasing trend in land surfacetemperature worldwide is the most obvious aspectof anthropogenic climate change. The global meantemperature has increased by about 0.8 °C since theindustrial revolution, whereby the largest warminghas occurred in Polar regions (see Section 2.2.2).Many other climate variables have changed as well.Observations show increases in ocean temperatureto depths of at least 3 000 m, in atmospheric watercontent, and in sea level since at least 1950. At thesame time, the Greenland ice sheets, Arctic seaice, mountain glaciers and snow cover in bothhemispheres are declining rapidly. Significantchanges have also been observed in precipitationamounts, ocean salinity and wind patterns. Changesin some weather and climate extremes have alsobeen detected in some regions since 1950, inparticular increases in daily temperature extremesand heat waves (IPCC, 2012).2.1.3 Future climate changeProjections of future climate change are derivedfrom simulations with general circulation modelsand regional climate models using differentemission scenarios for GHGs and aerosols(see Box 2.1 for further information). Thesemodels agree that past human activities willcontinue to warm the climate and raise sea levelsfor many decades to come. This future climatechange commitment is due to the long lifetime ofanthropogenic GHGs in the atmosphere (typicallydecades to centuries) and to the large inertia of theclimate system (in particular the oceans). However,the pace and magnitude of future climate changedepends on the level of global GHG emissionsand other human activities. The best estimatesof further warming (relative to the 1980–1999average) provided in the IPCC AR4 are 1.8 °C fora low emissions scenario (SRES B1) (Nakicenovicand Swart, 2000) and 4.0 °C for a high emissionsscenario (SRES A1FI). When uncertainties inclimate modelling are considered, the likely rangefor 21st century global warming based on the sixSRES marker emission scenarios extends to1.1–6.4 °C ( 32 ). For comparison, the difference inglobal mean temperature between the presentwarm phase and the coldest phase of the last iceage (around 22 000 years ago) is about 5–6 °C.Future warming of the Earth will affect otheraspects of the climate system as well, leadingto increasing sea levels, changing precipitationpatterns, and changes in weather and climateextremes. The spatial pattern of climate change inthe coming decades is expected to be largely similarto the pattern of recent changes, which shows aparticularly strong warming in high latitudes,increasing precipitation in most tropical and highlatitude regions, and decreasing precipitation inmost sub-tropical regions.( 32 ) Climate projections in the IPCC AR5 will be based on so-called representative concentration pathways (RCPs) rather than theSRES emissions scenarios. A summary of these simulations will only be publicly available in 2013 even though many individualresults are already available through the CMIP5 website (http://cmip-pcmdi.llnl.gov/cmip5/).Climate change, impacts and vulnerability in Europe 201251