Key statements from the Summary for Policymakers, WG1 contribution to the AR5 of IPCC

Transcription

1 Key statements from the Summary for Policymakers, WG1 contribution to the AR5 of IPCC Jochem Marotzke Max-Planck-Institut für Meteorologie KlimaCampus, Hamburg

2 Attribution of observed climate change to human influence Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system. {2 14} Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes (Figure SPM.6, Table SPM.1). { , 10.9} It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period. {10.3} 2

3 By including the carbon cycle in climate models, we can now link climate targets to a budget of total (cumulative) CO 2 emissions Cumulative emissions of CO 2 largely determine global mean surface warming by the late 21 st century and beyond {12.5}. Limiting the warming caused by anthropogenic CO 2 emissions alone with a probability of [ ] >66% to less than 2 C since the period , will require cumulative CO 2 emissions from all anthropogenic sources to stay [below] about 1000 GtC since that period. [About] 530 GtC was already emitted by {12.5} Note: Annual total CO 2 emissions were slightly above 10 GtC per year (corresponding to about 37 gigatonnes of CO 2 per year). [For any] given level of warming [ ], higher emissions in earlier decades imply lower emissions later. {12.5} 3

4 CMIP5 models reproduce long-term GMST over the 20 th century, but not the warming hiatus over the past 15 years WG1, AR5, Figure 9.8 4

5 Over , GMST trend is higher than observed in 111 out of the 114 available CMIP5 historical simulations GMST linear trends Length of histogram bar displays how many simulations show a trend in the range defined by the width of the bar Radiative forcing linear trends WG1, AR5, Box 9.2, Figure 1 5

6 It matters when you look observations show larger trend than models over Long-term trend reproduced by models. GMST linear trends Radiative forcing linear trends Length of histogram bar displays how many simulations show a trend in the range defined by the width of the bar WG1, AR5, Box 9.2, Figure 1 6

7 Observations and model simulations of surface temperature trends over past 15 years: AR5 assessment The observed reduction in surface warming trend over the period as compared to the period , is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence) There is medium confidence that internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of internal variability. WG1, AR5, Box 9.2 7

8 Conclusions: the recent surface-warming hiatus Recent surface-warming hiatus poses a fascinating scientific challenge; it forces us to think across entire climate system. By contrast, the hiatus is largely irrelevant to the physics of anthropogenic climate change (not the communication, though!). Timescale is too short and hence dominated by natural (forced and internal) fluctuations. Role of climate sensitivity very minor. Reminder: warming continues in practically all components of the climate system except the surface. We find a very robust connection between radiative imbalance (greenhouse gases!) and observed increase of energy content of climate system. Thank you for your attention! 8