WORLD METEOROLOGICAL ORGANIZATION
EXECUTIVE COUNCIL
PANEL OF EXPERTS ON POLAR OBSERVATIONS,
RESEARCH AND SERVICES
Fifth session
Wellington, New Zealand, 25 – 28 February 2014
EC-PORS-5/ INF. 33
Date: 2 January 2014
AGENDA ITEM: 4
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;

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:

RCP 8.5: 8.5 w/m 2 by 2100 but rising

RCP 6.0: 6.0 w/m 2 with stabilization after 2100

RCP 4.5: 4.5 w/m 2 with stabilization after 2100

RCP 2.6: 2.6 w/m 2 with a peak of 3 w/m 2 before 2100 then declining to 2.6 w/m 2
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 2 o C, 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-CO
2
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 0.8
5°C since the beginning of the 20 th 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 21 st 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 CO
2.
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.

EC-PORS-5/INF. 33, Page 4/4
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.