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CLIMATE CHANGE

SCIENCE AND ITS

RELEVANCE TO

POLICy By Dr. R.K. Pachauri

he Intergovernmental Panel on Climate Change (IPCC) is now in the than the mean rate during the previous two millennia.
fnal stages of completing its Fifth Assessment Report (AR5). The frst Over the period 1901–2010, global mean sea level rose
Tcontribution to this report prepared by Working Group I (WG-I) of the by 0.19 (0.17 to 0.21) m. All these changes are taking
IPCC, covering the physical science basis of climate change, was completed place essentially because human actions have been
and released in September 2013. This has been a major effort to advance contributing at a rapidly increasing rate to enhancing
our knowledge substantially beyond what we were able to provide in the the atmospheric concentrations of major greenhouse
Fourth Assessment Report (AR4) released in 2007. In several respects the gases (GHGs). The atmospheric concentrations of
WG-I report as part of the AR5 has confrmed and strengthened several of carbon dioxide (CO2), methane, and nitrous oxide have
the fndings arrived at in the AR4. The WG-I report involved 209 Lead Authors increased to levels unprecedented in at least the last
and 50 Review Editors drawn from 39 countries. It also beneftted from 800,000 years. CO2 concentrations have increased by
600 contributing authors from 32 countries. In the process of completing 40% since pre-industrial times, primarily from fossil fuel
this report the authors have actually studied and cited over 9200 scientifc emissions and secondarily from net land use change
publications, almost two-thirds of which have been published after 2007. emissions. The ocean has absorbed about 30% of the
In other words this report contains the results of a substantial amount of emitted anthropogenic carbon dioxide, causing ocean
new research and knowledge which have appeared in publications after the acidifcation.
previous report.
“

The IPCC functions in an open and transparent manner, and at various stages of The IPCC produces
the process of completing an assessment the authors seek expert reviews as well assessments which are
as reviews from governments on various versions of the drafts of the report before
fnalisation. In the case of the WG-I report 54,677 comments were received from policy relevant but not
1089 expert reviewers drawn from 55 countries, and 38 governments provided their
comments as part of this process. Some of the major fndings of the report include policy prescriptive ”
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 In the AR5, scenarios for the future have been defned
amount of snow and ice have diminished, sea level has risen, and the concentrations in terms of specifc Representative Concentration
of greenhouse gases (GHGs) have increased. Most signifcantly each of the last three Pathways (RCPs). If we look at projections for the future
decades has been successively warmer at the Earth’s surface than any preceding we fnd that global surface temperature change for the
decade since 1850. In the Northern Hemisphere, 1983–2012 was likely the warmest end of the 21st century is likely to exceed 1.5°C relative
30-year period of the last 1400 years. 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
It was also found that ocean warming dominates the increase in energy stored likely than not to exceed 2°C for RCP4.5. Warming will
in the climate system, accounting for more than 90% of the energy accumulated continue beyond 2100 under all RCP scenarios except
between 1971 and 2010. It is virtually certain that the upper ocean (0−700 m) warmed RCP2.6. Warming will continue to exhibit interannual-
from 1971 to 2010, and it likely warmed between the 1870s and 1971. In the AR4 to-decadal variability and will not be regionally uniform.
there was a substantial amount of detail provided on the manner and rate at which Also, the global ocean will continue to warm during the
bodies of ice across the world were melting. In the AR5 we have stated that over 21st century. Heat will penetrate from the surface to the
the last two decades, the Greenland and Antarctic ice sheets have been losing deep ocean and affect ocean circulation. Further, it is
mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and very likely that the Arctic sea ice cover will continue to
Northern Hemisphere spring snow cover have continued to decrease in extent. The shrink and thin and that Northern Hemisphere spring
IPCC had assessed in the AR4 that melting of ice across the globe and thermal snow cover will decrease during the 21st century as
expansion of the oceans was resulting in sea level rise. However, in the AR5 it has global mean surface temperature rises. Global glacier
been stated that the rate of sea level rise since the mid-19th century has been larger volume will further decrease. Global mean sea level rise
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