Info Service on Climate Change (Sept13/07)
IPCC meeting grapples with key issues
Stockholm, 30 Sept (Meena Raman) – Prior to the approval by governments of the Intergovernmental Panel on Climate Change (IPCC)’s ‘Summary for Policymakers’ of Working Group I on the ‘Physical Science Basis’, intense debates took place on several key issues.
Among the issues which were deliberated during the meeting of Working Group 1 (WG1) which began on 23 September in Stockholm, Sweden, included: the “pause” in the warming trend for the 15 year period from 1998 to 2012; the lack of temperature data for the pre-industrial period from 1750; the evaluation of climate models and their reliability in reproducing observations; the emission cuts needed by 2050 relative to 1990 and the carbon budget remaining in relation to temperature limits. Also addressed was the issue of ‘geo-engineering’.
Concerns raised by governments were in some cases referred to contact groups or informal consultations for resolution. The Summary for Policy Makers (SPM) was approved by governments on 27 September.
Below are highlights from some of the discussions.
‘Pause’ in warming
When the initial draft of the SPM (dated 7 June 2013) was made available to governments, a key issue of concern was reference to “the global mean surface temperature trends, where the rate of warming over the past 15 years (1998-2012) was smaller than the trend since 1951”. (This has been referred to in the media as a ‘pause’ in the warming). “The mean rate of warming for the 15 years was 0.05 degree C compared to the mean rate per decade (1951-2012) of 0.12 degree C,” according to the SPM.
to Thomas Stocker (Switzerland), who is the co-chair of WG1, concerns
were raised by some governments regarding the scientific basis of
looking at short term periods. He said that an equal number of governments
felt that this (15 years) is an important period of time on which
the IPCC has to make a statement on a scientific basis.
Germany stressed that showing trends alone is misleading and there is need to also show the context. It said that the impression given is that warming is going to slow down or is halted. It wanted reflection of the trends for the following 15 years. This was however not agreed to.
The US emphasised the need to address inter-annual variability. It also wanted the mention of 1998 being an El Niño event. The US said that the rate of warming since the late 1990s is very sensitive to choice of start dates.
(An El Niño is a temporary change in the climate of the Pacific Ocean, in the region around the equator where effects are seen in both the ocean and atmosphere, generally in the Northern Hemisphere winter. Typically, the ocean surface warms up by a few degrees Celsius).
China said that it is important to reflect the temperature changes which are a fact and to also reflect the short term uncertainty. Saudi Arabia stressed the need to state the internal climate variability.
The final paragraph agreed to is as follows: “In addition to robust multi-decadal warming, global mean surface temperature exhibits substantial decadal and interannual variability. Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to +0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade).”
A footnote was added that reads: “Trends for 15-year periods starting in 1995, 1996, and 1997 are 0.13 [0.02 to 0.24], 0.14 [0.03 to 0.24], 0.07 [–0.02 to 0.18] °C per decade, respectively.”
Lack of temperature data for pre-industrial period
During the discussion on ‘observed changes to the climate system’ relating to the atmosphere, an issue which arose was the reference year used. In the initial draft circulated to the governments in June, the paragraph of concern read as follows:
“The globally averaged combined land and ocean surface temperature data show an increase of 0.89 [0.69 to 1.08] °C over the period 1901-2012. Over this period, almost the entire globe has experienced surface warming.”
The Netherlands said that the SPM did not make clear why 1901 was chosen, and asked why different sections used different reference years. It referred to negotiations under the United Nations Framework Convention on Climate Change (UNFCCC), where the reference year is 1990 and there is agreement to hold the increase in global average temperature to below 2 °C above pre-industrial levels. It suggested the use of the period 1850-2012.
This was supported by Norway, which added that it is important to peg the reference year to the pre-industrial level of 1850. St. Lucia also agreed with Netherlands that it is important to have a consistent reference period from 1850 for policy makers.
In response, the authors (who are scientists) said there are several reasons for choosing 1901 to 2012, as they wanted to highlight the long term change and that before 1901, data coverage and quality was sparse, hence the focus on 1901. The other reason is that comparison with previous SPMs (of previous IPCC assessments) is now possible because they also start in 1901. Stocker said that 1901 is a conscious choice because of the availability of the datasets.
The Netherlands insisted that for policymakers, 1850 is the relevant period for the 2 degree C. Germany also said that the temperature estimate between 1850 to 1901 is important. Stocker in response said the temperature figure for the period 1880-2012 could be given as this is the result from three datasets.
The United Kingdom also wanted a reference to the pre-industrial period as this is what is referred to in the Convention. Germany referred to the IPCC’s 4th Assessment Report (AR4) where the total temperature increase was from 1850-1999 and from 2001-2005. It wanted the authors to provide figures that allow for direct comparability.
India said that it would be appropriate to give the same period and give direct comparison with AR4 time periods. If there are data limitations, qualifying statements about the limitations of that figure should be provided.
Stocker also clarified that on the use of the term ‘pre-industrial’, that term is used to refer to the period that goes back to 1750. So, the period from 1850 is what we would call as early instrumental period, and then from 1900, is the period with instrumental observations.
The authors reiterated that the datasets on global scale go back only to 1850; there are sparse observations that go back to 1750 on a global scale.
The UK said that there must be some way of addressing the issue of the ‘pre-industrial’ period. Germany said that there could be text to say “pre-industrial, approximated by 1850-1900”. Brazil said that the pre-industrial period refers back to 1750, and it did not think one can use 1850 as an approximation for that as there is a difference of 100 years.
Given the lack of consensus in this regard, an informal consultation group was formed to discuss the issue.
Following the informal consultation, what was finally agreed to is as follows: “The globally averaged combined land and ocean surface temperature data as calculated by a linear trend, show a warming of 0.85 [0.65 to 1.06] °C, over the period 1880–2012, when multiple independently produced datasets exist. The total increase between the average of the 1850–1900 period and the 2003–2012 period is 0.78 [0.72 to 0.85] °C, based on the single longest dataset available.”
A footnote was added at the end of the paragraph which reads: “Both methods presented in this bullet were also used in AR4. The first calculates the difference using a best fit linear trend of all points between 1880 and 2012. The second calculates the difference between averages for the two periods 1850 to 1900 and 2003 to 2012. Therefore, the resulting values and their 90% uncertainty intervals are not directly comparable.”
Evaluation of climate models
The SPM which was approved states that “understanding recent changes in the climate system results from combining observations, studies of feedback processes, and model simulations. Evaluation of the ability of climate models to simulate recent changes requires consideration of the state of all modelled climate system components at the start of the simulation and the natural and anthropogenic forcing used to drive the models. Compared to AR4, more detailed and longer observations and improved climate models now enable the attribution of a human contribution to detected changes in more climate system components”.
In the SPM, under the section on ‘understanding recent changes in the climate system’, one aspect relates to the ‘evaluation of climate models’.
In this regard, in the June version of the draft of the SPM, there was a statement that “there is very high confidence that models reproduce the more rapid warming in the second half of the 20th century and cooling immediately following large volcanic eruptions. Models do not generally reproduce the observed reduction in surface warming trend over the last 10-15 years. There is medium confidence that this difference between models and observations is to a substantial degree caused by unpredictable climate variability, with possible contributions from inadequacies in the solar, volcanic, and aerosol forcings used by the models and, in some models, from too strong a response to increasing green-house gas forcing.”
Following comments by various governments prior to the meeting in Stockholm, the authors amended the above paragraph to read as follows: “The long-term climate model simulations show a trend in global-mean surface temperature from 1951- 2012 that agrees with the observed trend, despite differences between simulated and observed trends 25 over the past 10–15 years (very high confidence).”
“The observed reduction in surface warming trend over the period 1998–2012 as compared to the period 1951–2012, 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). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. 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.”
(Levels of confidence is the IPCC indicator for the robustness of conclusions drawn from the data and literature that is assessed.)
Venezuela sought clarification over the ‘very high confidence’ judgement by the authors while China said that the paragraphs above were new and are supposed to be an evaluation of the climate models. It wanted to know the advantages and disadvantages of the models and said the June version of the SPM draft had this. Similar sentiments were expressed by Saudi Arabia.
The United States said the paragraphs did relate to the evaluation of the models and it is about observations in a broader context and not just 10 to15 years. Switzerland said that one could not test models in 15 years.
Saudi Arabia, referring to the Technical Summary of WG 1, wanted the following sentence added: “Most, though not all, models overestimate the observed warming trend in the tropical troposphere over the last 30 years, and tend to underestimate the long-term lower-stratospheric cooling trend.”
(The Technical Summary is drawn from the full report of the Working Group.)
Following consultations with the authors, the following compromise was reached at around 2 am on Thursday, 26 September: “The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence). There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years (e.g., 1998 to 2012).”
“The observed reduction in surface warming trend over the period 1998–2012 as compared to the period 1951–2012, 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). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. 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. There may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing (dominated by the effects of aerosols.)”
When the meeting resumed on the final day at around 8 am, Germany requested a reconsideration of the last sentence in the paragraph above as it had concerns. Since agreement on the above paragraph had been gavelled, Stocker did not want a re-opening of the agreed text.
Emission reductions required by 2050
Under the discussion on ‘Future global and regional climate change’ the SPM which was approved states that “Projections of changes in the climate system are made using a hierarchy of climate models…These models simulate changes based on a set of scenarios of anthropogenic forcings. A new set of scenarios, the Representative Concentration Pathways (RCPs), was used for the new climate model simulations…In all RCPs, atmospheric CO2 concentrations are higher in 2100 relative to present day as a result of a further increase of cumulative emissions of CO2 to the atmosphere during the 21st century.” (The four RCPs used are RCP2.6, RCP4.5, RCP6, and RCP8.5, and named after a possible range of radiative forcing values in the year 2100).
One aspect that was discussed in this regard related to ‘carbon and other biogeochemical cycles’.
Member States were asked to consider the following statement: “Based on Earth System Models, following RCP2.6 requires by 2050 an average emission reduction of 50% (range 14% to 96%) relative to 1990 emissions, and requires, about as likely as not, sustained net removal of CO2 from the atmosphere by the end of the 21st century.”
Germany wanted additional information on the emissions pathway beyond 2050 and asked for a figure for emissions reduction in 2080. China expressed concerns as to how one single figure is obtained and asked how the figure 50% figure was obtained when this is not what the ranges indicate. It also said that no information has been provided from other scenarios and only one scenario in RCP 2.6 is considered.
Saudi Arabia said that members are talking about projections that would resonate with policy makers. It can be misleading to talk about a range from 14% to 96%. It wanted the entire paragraph deleted. Russia also had similar concerns.
A contact group was formed to address this issue. Following the discussions, the following paragraph was agreed to: “By 2050, annual CO2 emissions derived from Earth System Models following RCP2.6 are smaller than 1990 emissions (by 14% to 96%)... By the end of the 21st century, about half of the models infer emissions slightly above zero, while the other half infers a net removal of CO2 from the atmosphere.”
In the discussion on ‘climate stabilisation’, Member States were asked to consider the following paragraph:
“Limiting the warming caused by anthropogenic CO2 emissions alone to likely less than 2°C relative to pre-industrial, will require cumulative CO2 emissions from all anthropogenic sources to stay below about 1000 GtC since the beginning of the industrial era. This amount is reduced to about 800 GtC when accounting for non-CO2 forcings as in RCP2.6. An amount of 545 [460 to 630] GtC, was already 16 emitted by 2011.”
China raised concerns that the cumulative reductions required to stay below 2 degree C are not consistent with the figures in the underlying assessment report of WG1 which reflect that emission reductions should be in the range of 800-2500 GtC between 1750 to 2100.
The US said that the IPCC is supposed to be policy neutral and the information needs to be presented in a neutral way.
Informal consultations were held in this regard and the text which was agreed is as follows:
“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.”
Another issue in the SPM concerns ‘geo-engineering’. Member States were asked to consider the following statement:
“Methods that aim to deliberately alter the climate system to counter climate change, termed geo-engineering, have been proposed. Limited evidence precludes a comprehensive quantitative assessment of both Solar Radiation Management (SRM) and Carbon Dioxide Removal (CDR) and their impact on the climate system. CDR methods have biogeochemical and technological limitations to their potential on a global scale. There is insufficient knowledge to quantify how much CO2 emissions could be partially offset by CDR on a century timescale. Modelling indicates that SRM methods, if realizable, have the potential to substantially offset a global temperature rise, but they would also modify the global water cycle, and would not reduce ocean acidification. If SRM were terminated for any reason, there is high confidence that global surface temperatures would rise very rapidly to values consistent with the greenhouse gas forcing. CDR and SRM methods carry unintended side effects and long-term consequences on a global scale.”
The US raised concerns over the last sentence, as it felt that it overstated the state of science in that the side effects are known and that they were unintended. It wanted reflection of the uncertainties in understanding. The authors responded that there is evidence of the side effects but were prepared to delete the word ‘unintended’ from the last sentence. This led to agreement of the text.