Download IPCC Fifth Assessment Report (AR 5)

WORLD METEOROLOGICAL ORGANIZATION
EC-PORS-5/ INF. 33
EXECUTIVE COUNCIL
PANEL OF EXPERTS ON POLAR OBSERVATIONS,
RESEARCH AND SERVICES
Fifth session
Date: 2 January 2014
AGENDA ITEM: 4
Wellington, New Zealand, 25 – 28 February 2014
INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE
FIFTH ASSESSMENT REPORT
WORKING GROUP I: THE PHYSICAL SCIENCE BASIS
ISSUES TO BE DISCUSSED: The Summary for Policymakers of the contribution of Working
Group I to the IPCC Fifth Assessment Report was approved, and the full report accepted, on 27
September 2013. The report contains many references to the changing state of the cryosphere
including permafrost, glaciers and sea-ice.
DECISIONS/ACTIONS REQUIRED:
1. The Panel is invited to review the relevant findings of IPCC Working Group I and
consider how best to employ the findings of AR 5 in its relationship with stakeholders,
work-planning and communications opportunities.
REFERENCES: The finalized version of the Summary for Policymakers was published on
11 November 2013 and is available for download at http://www.ipcc.ch/
EC-PORS-5/INF. 33, Page 2/4
1. Context
1.1 Working Group I: The Physical Science Basis delivered its report on September 27, 2013.
The timeline for the complete Fifth Assessment Report is as follows;
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Working Group II: Impacts, Adaptation and Vulnerability - March 29, 2014
Working Group III: Mitigation of Climate Change - April 11, 2014
Synthesis Report: October 31, 2014
1.2 Overall, scientific understanding of historical climate change and projection of future
climates is incrementally more robust. Global average temperature has increased about 0.9 °C
over the period 1901-2012. Extending the record back to 1880 reduces the linear warming trend
to 0.85 °C (indicating simply that slight differences in trend estimates will occur with different
start dates.) This represents warming averaged over the whole of the surface of the Earth, using
estimates from measurements of surface air temperature over land combined with sea surface
temperature measurements. The most recent decade, 2000-2010, was the warmest on record.
1.3 For the Fifth Assessment Report of IPCC, new climate change projections are based on a
set of four new scenarios called Representative Concentration Pathways (RCPs). As a set,
these four RCPs span a range of assumptions about future controls on greenhouse gas (and
other) emissions. The lowest RCP represents a very aggressive GHG mitigation scenario aimed
at limiting global warming to about 2°C, while the highest RCP corresponds to minimal effort to
reduce GHG emissions this century. This differentiates this set of scenarios from those used in
previous IPCC assessment reports, which were ‘business-as-usual’ scenarios assuming no
policies to address climate change.
1.4 The four RCPs are identified by their associated warming effect (radiative forcing; measured
in units of watts per metre squared) in the year 2100:
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RCP 8.5: 8.5 w/m2 by 2100 but rising
RCP 6.0: 6.0 w/m2 with stabilization after 2100
RCP 4.5: 4.5 w/m2 with stabilization after 2100
RCP 2.6: 2.6 w/m2 with a peak of 3 w/m2 before 2100 then declining to 2.6 w/m2
1.5 Despite the record warmth of recent years, the rate of global warming (the increase in
temperature over time) has slowed over the last 15 years in comparison to the longer term
warming rate. Over the period 1998-2012, the rate of warming has been ~0.05 °C per decade
compared to ~0.08 °C per decade over the period 1901-2012.
1.6 This recent “warming hiatus” – a phrase being used to refer to this pause in the long term
warming trend – can be explained by a combination of natural and anthropogenic factors. This
warming hiatus does not fundamentally change projections of future global warming which are
largely driven by human emissions of greenhouse gases. A warming hiatus also occurred in the
period between 1940-1970.
1.7 To keep warming below 2oC, the Copenhagen Accord target, with a likelihood of 66% or
greater, it is estimated that cumulative emissions of carbon dioxide need to be limited to about
one thousand gigatonnes of carbon (1000GtC), about half of which (531GtC [446 to 616] was
already emitted by 2011). This upper limit is reduced to about 800GtC when accounting for the
warming contributions of emissions of non-CO2 forcers (such as methane, ozone and nitrous
EC-PORS-5/INF. 33, Page 3/4
oxide), as in the RCP 2.6 scenario. Emissions of these other GHGs and air pollutants (e.g. SOx
and NOx) decrease over time in RCP2.6.
2. Key Scientific Findings from Working Group I Report
2.1 Globally, each of the last three decades has been warmer than all preceding decades since
1850. Global temperature has increased approximately 0.85°C since the beginning of the 20th
century -- almost half way to the 2°C global warming target.
2.2 Increasing carbon dioxide in the atmosphere has been the dominant driver of global
warming over the industrial era. Cooling from aerosols (primarily sulphate aerosols) has offset
some of the warming that would otherwise have occurred.
2.3 Several future greenhouse gas emission scenarios have been studied, spanning the range
from rapid mitigation to ‘business as usual’. These scenarios take the form of Representative
Concentration Pathways (RCPs), and range from a mitigation scenario (RCP2.6), to two
stabilization scenarios (RCP4.5 and 6.0), to a scenario with very high greenhouse gas
emissions (RCP8.5). Global surface temperature 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 interannual-to-decadal variability and will not be regionally uniform.
2.4 Owing to the long lifetime of carbon dioxide in the atmosphere, global temperature will
remain approximately constant for many centuries, even after emissions of carbon dioxide have
stopped. This represents a substantial multi-century climate change commitment created by
past, present and future emissions of CO2.
3. Statements of relevance to EC PORS
3.1 In general, over recent decades or longer, there is evidence that all elements of the
cryosphere (snow, ice and frozen ground) are warming or decreasing in extent. Measurements
show significant decreases in Northern Hemisphere snow cover extent, in Arctic sea ice extent,
increases in permafrost temperatures and reductions in permafrost thickness and extent. The
addition of water from melting land ice to the ocean, along with expansion of ocean water due
to warming, has caused sea level to rise by about 19cm since 1900
3.2 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.
3.3 Annual mean Arctic sea ice extent decreased over the period 1979-2012 at a rate estimated
to be (with 90-100% likelihood) 3.5 to 4.1% per decade. The most rapid decreases have been in
the summer and autumn. Arctic sea ice extent in 2013 was higher than the record low set in
2012. However, the overall long-term trend continues to be declining sea ice extent in the Arctic.
Arctic sea ice extent in September 2013 (5.3 million km2) was higher than the record low set in
September 2012 (3.4 million km2). However, the long-term trend continues to show declining
sea ice extent for the Arctic. Over the period of 1979–2013, the trend in September Arctic sea
ice extent is -13.7% per decade. 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.
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Snow, ice and permafrost are in decline, consistent
with observed warming.
Decreases in Arctic sea ice extent have been most
rapid in summer
3.4 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.
4. Conclusions
The fundamental conclusions have changed little since the IPCC’s Fourth Assessment. This is
a testament to the overwhelming consensus within the scientific community. EC PORS is invited
to consider the implications of these findings toward its deliberations on the future workplan, for
its outreach to stakeholders and other communication opportunities.