Chief of UN climate science body says negotiators should base their discussions on the scientific findings of recent IPCC AR5 report
(Pic: Tim J Keegan)
The Intergovernmental Panel on Climate Change (IPCC) is now in the final stages of completing its Fifth Assessment Report (AR5).
The first contribution to this report prepared by Working Group I (WG-I) of the IPCC, covering the physical science basis of climate change, was completed and released in September 2013. This has been a major effort to advance our knowledge substantially beyond what we were able to provide in the Fourth Assessment Report (AR4) released in 2007.
In several respects the WG-I report as part of the AR5 has confirmed and strengthened several of the findings arrived at in the AR4. The WG-I report involved 209 lead authors and 50 review editors drawn from 39 countries. It also benefitted from 600 contributing authors from 32 countries.
In the process of completing this report the authors have actually studied and cited over 9,200 scientific publications, almost two-thirds of which have been published after 2007. In other words this report contains the results of a substantial amount of new research and knowledge which have appeared in publications after the previous report.
Unequivocal warming
The IPCC functions in an open and transparent manner, and at various stages of the process of completing an assessment the authors seek expert reviews as well as reviews from governments on various versions of the drafts of the report before finalisation.
In the case of the WG-I report 54,677 comments were received from 1089 expert reviewers drawn from 55 countries, and 38 governments provided their comments as part of this process. Some of the major findings of the report include the statement that warming of the climate system is unequivocal, and that since the 1950s many of the observed changes have been unprecedented over decades to millennia.
It was observed that the atmosphere and oceans have warmed, the amount of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases (GHGs) have increased.
Most significantly each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. In the Northern Hemisphere, 1983–2012 was likely the warmest 30-year period of the last 1400 years.
Marine impacts
It was also found that ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010. It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971.
In the AR4 there was a substantial amount of detail provided on the manner and rate at which bodies of ice across the world were melting. In the AR5 we have stated that over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent.
The IPCC had assessed in the AR4 that melting of ice across the globe and thermal expansion of the oceans was resulting in sea level rise. However, in the AR5 it has been stated that the rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia. Over the period 1901–2010, global mean sea level rose by 0.19 (0.17 to 0.21) m.
All these changes are taking place essentially because human actions have been contributing at a rapidly increasing rate to enhancing the atmospheric concentrations of major greenhouse gases (GHGs). The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years.
CO2 concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land use change emissions. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.
Projections
In the AR5, scenarios for the future have been defined in terms of specific Representative Concentration Pathways (RCPs).
If we look at projections for the future we find that global surface temperature change for the end of the 21st century is likely to exceed 1.5°C relative to 1850 to 1900 for all RCP scenarios except RCP2.6. It is likely to exceed 2°C for RCP6.0 and RCP8.5, and more likely than not to exceed 2°C for RCP4.5. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6.
Warming will continue to exhibit inter-annual to decadal variability and will not be regionally uniform. Also, the global ocean will continue to warm during the 21st century. Heat will penetrate from the surface to the deep ocean and affect ocean circulation.
Further, it is very likely that the Arctic sea ice cover will continue to shrink and thin and that Northern Hemisphere spring snow cover will decrease during the 21st century as global mean surface temperature rises. Global glacier volume will further decrease.
Global mean sea level rise for 2081−2100 relative to 1986–2005 will likely be in the ranges of 0.26 to 0.55 m for RCP2.6, 0.32 to 0.63 m for RCP4.5, 0.33 to 0.63 m for RCP6.0, and 0.45 to 0.82 m for RCP8.5. For RCP8.5, the rise by the year 2100 is 0.52 to 0.98 m, with a rate during 2081–2100 of 8 to 16 mm yr–1.
These ranges are derived from specific climate projections in combination with process-based models and literature assessment of glacier and ice sheet contributions.
Carbon budget
In the case of the carbon cycle it was found that climate change will affect carbon cycle processes in a way that will exacerbate the increase of CO2 in the atmosphere.
Further uptake of carbon by the ocean will increase ocean acidification. Human actions in the past have limited the choices now available to us for reduction of GHG emissions in the future.
For instance, limiting the warming caused by anthropogenic CO2 emissions alone with a probability of >33%, >50%, and >66% to less than 2°C since the period 1861–1880, will require cumulative CO2 emissions from all anthropogenic sources to stay between 0 and about 1560 GtC, 0 and about 1210 GtC, and 0 and about 1000 GtC since that period respectively.
These upper amounts are reduced to about 880 GtC, 840 GtC, and 800 GtC respectively, when accounting for non-CO2 forcings as in RCP2.6. An amount of 531 (446 to 616) GtC, was already emitted by 2011.
Further research
As part of the AR5 three other reports are due to be completed and released by the IPCC. The Working Group-II report due in March 2014 would deal with impacts, adaptation and vulnerability and the Working Group-III report due to be completed in April 2014 would assess mitigation measures.
Finally, the Synthesis Report (SYR) which would be completed in October 2014 would essentially synthesise the material from all the three Working Group reports as well as special reports which have been brought out by the IPCC since the AR4.
The IPCC produces assessments which are policy relevant but not policy prescriptive. The SYR would provide a comprehensive assessment of policy relevance in relation to actions that need to be taken to deal with the challenge of climate change.
But the reinforcement and elaboration of the findings of the AR4 as contained in the AR5 WG-I report tell us clearly that human society already has adequate scientific knowledge on the basis of which action can be taken to deal with the challenge of climate change.
Dr Rajendra Pachauri is Director-General of the Energy & Resources Institute (TERI) and chairman of the Intergovernmental Panel on Climate Change (IPCC)