Irreversible now

The latest report from the IPCC states in stronger language than ever before that human activity has set the earth on an irreversible path to climate change, the effects of which will persist for centuries even if carbon dioxide emissions are curtailed now.

(a) Observed global mean combined land and ocean surface temperature anomalies, from 1850 to 2012, from three data sets. Top panel: annual mean values; bottom panel: decadal mean values, including the estimate of uncertainty for one data set (black). Anomalies are relative to the mean of 1961-1990. (b) Map of the observed surface temperature change from 1901 to 2012 derived from temperature trends determined by linear regression.

“IT is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” This is the latest assessment of the Intergovernmental Panel on Climate Change (IPCC) in its Summary for Policymakers (SPM) of the yet-to-be-released underlying scientific and technical assessment of its Working Group 1 (WG1) titled “Climate Change: the Physical Science Basis”. The SPM was released on September 27 in Stockholm. (The italicised phrase above and other similar probabilistic qualifiers mean specific quantitative values in IPCC assessments. Extremely likely indicates an assessment with a 95-100 per cent probability. Other phrases of likelihood have been used in italics in the article. For corresponding probability values, see the box.)

“Human influence on the climate system is clear…. It has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level [MSL] rise, and in changes in some climate extremes,” says the document. According to the report, more detailed and longer observations and improved climate models compared with earlier assessments of the IPCC have now enabled a much better understanding of the climate system and have led to a clearer attribution of the human contribution to detected changes in more climate system components. Although the main conclusion regarding the impending climate crisis was already there in the IPCC’s earlier assessments, the present one brings home the same message with much stronger evidence and more robust methodologies.

The draft of the WG1 report has already been made public on the IPCC’s website with the disclaimer that it has yet to be approved in detail and with the condition that it should not be cited, quoted or distributed. The final WG1 report will be issued in January 2014. A total of 209 lead authors and 50 review editors from 39 countries and more than 600 contributing authors from 32 countries have contributed to the WG1 report.

This and the assessments of the other two working groups, WG2 and WG3, will together constitute the Fifth Assessment Report (AR5) of the IPCC. The reports of WG2 and WG3 look respectively at the impacts of climate change on human and natural systems and ways to meet the challenge of climate change. These will be released in March and April 2014. The IPCC AR5 cycle will conclude with the publication of its Synthesis Report in October 2014.

The Fourth Assessment Report (AR4) was released in 2007. The main conclusion in it ( Frontline , March 9, 2007), based on increases in global average air and ocean temperatures, the widespread melting of snow and ice, and the rising global MSL, was that the warming of the climate system was unequivocal. It said that most of the observed increase in globally averaged temperatures since the mid-20th century was very likely due to the observed increase in anthropogenic greenhouse gas (GHG) concentrations. In the AR3, released in 2001, this causative link was estimated to be only likely .

The conclusion in the AR5, too, is that global warming remains unequivocal. “Since the 1950s,” says the SPM of the AR5, “many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of GHGs have increased.”

But, more specifically, the global warming caused by an increase in anthropogenic GHG concentrations is now assessed to be extremely likely . “It is extremely likely ,” says the SPM, “that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in GHG concentrations and other anthropogenic forcings together.”

According to the report, the likely estimates of the different contributions to global warming are as under:

•GHGs: 0.5-1.3 °C

•Cooling effect of aerosols: −0.6-0.1 °C

•Natural forcings: −0.1-0.1 °C

•Internal variability: −0.1-0.1 °C

“Together these assessed contributions are consistent with the observed warming of approximately 0.6 °C to 0.7 °C over this period,” says the report.

Robust warming trend The globally averaged combined land and ocean surface temperature, as calculated by a linear trend, shows a warming of 0.85 °C over the period 1880-2012 on the basis of multiple independently generated datasets, according to the SPM. Quantifying the uncertainty in this estimate, the report gives a 90 per cent probability that the temperature rise lies in the range 0.65 to 1.06 °C. From another perspective, the report also gives an estimate of the total increase in temperature between the average of the period 1850-1900 and that of 2003-12 to be 0.78 °C, but this is based on the single longest dataset available. The 90 per cent uncertainty interval for this estimate is 0.72-0.85 °C. While these are not strictly comparable, they give an indication of the warming that has occurred in the last nearly 150 years. The AR4 too had used similar methodologies for its estimate and it gave an increase of 0.74 °C over the 100-year period 1906-2005.

In each of the past three decades, the earth’s surface has been successively warmer than any preceding decade since 1850. In the northern hemisphere, 1983-2012 was likely the warmest 30-year period in the past 1,400 years. The report makes further conclusions on the atmosphere component of the climate system. “It is virtually certain, ” the SPM says, “that, globally, the troposphere has warmed since the mid-20th century.”

However, the report points out that as against this robust multi-decadal warming trend surface temperature does show substantial decadal and inter-annual variability. And because of this natural variability, trends based on short records are not, in general, applicable for the long term. Climate sceptics have made much of this so-called “hiatus” in the warming trend in the 15 year-period from 1998-2012 when the rate of warming was just 0.05 °C per decade, with the 90 per cent uncertainty interval being −0.05-0.15 °C compared with the warming rate of 0.12 °C per decade calculated since 1951. Trends for the 15-year periods starting in 1995, 1996 and 1997, according to the report, are 0.13, 0.14 and 0.07 °C per decade respectively, which show that short-term trends are very sensitive to the start and end dates. Given the 90 per cent uncertainty intervals of these short-period trends, sceptics have been interpreting the data to suit their perspective on climate.

Continental-scale surface temperature reconstructions have shown, with high confidence , that there were multi-decadal periods during the Medieval Climate Anomaly (years 950 to 1250) when some regions of the globe were as warm as in the late 20th century. But these regional warm periods did not occur as coherently across regions as the warming in the late 20th century. (The AR5 has adopted two different metrics to describe the uncertainty in the evidence and conclusions drawn therefrom. Besides probability level, confidence level, as above, has also been used as a measure, which is expressed with five qualifiers: very low, low, medium, high and very high. Increasing levels of confidence imply increasing levels of evidence and agreement.) This implies that, unlike the medieval period, the present warming is irreversible. Indeed, as the co-chair of the IPCC, Thomas Stocker, said at the release of the SPM: “As a result of our past, present and expected future emissions of CO [carbon dioxide], we are committed to climate change, and effects will persist for many centuries even if emissions of CO stop.”

Since 1950, many extreme weather and climate events have been observed. It is, the report says, very likely that the number of cold days and nights has decreased and that the number of warm days and nights has increased on a global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. It is also very likely that the observed changes are due to human contributions, the report has assessed. There are likely more land regions where the number of heavy rainfall events has increased than where it has decreased.

Global warming implies that the energy stored in the climate system has increased. But 90 per cent of the energy accumulated during 1971-2010 is in the oceans, the report has assessed with high confidence . More than 60 per cent of this energy increase in the climate system is stored in the upper ocean (0-700 metres) and about 30 per cent below 700 m. Also, it is virtually certain that the upper ocean warmed during 1971-2010, and it is likely that it also warmed between the 1870s and 1971. On a global scale, the warming is largest near the surface, and the upper 75 m warmed by 0.11 °C per decade during 1971-2010, says the report. It also notes that since the AR4, the confidence in assessing this change has greatly increased with the vast improvement in the upper-ocean temperature records.

Melting ice sheets The cryosphere, the frozen part of the earth’s surface, which includes the polar ice caps, continental ice sheets, mountain glaciers, sea ice, snow cover, lake and river ice, and permafrost (perennially frozen ground wherever temperatures remain below 0 ºC for several years), is the third chief component of the climate system. Over the last two decades, the Greenland and Antarctic ice sheets—the only two ice sheets in the present age—have been losing mass, glaciers worldwide have continued to shrink, and Arctic sea ice and northern hemisphere spring snow cover have continued to decrease in extent, the report says with high confidence .

The average rate of ice loss every year from glaciers around the world, excluding those on the periphery of ice sheets, when assessed over the two periods of 1971-2009 and 1993-2009, shows an increase by about 22 per cent. This indicates that while the phenomenon of mass loss from glaciers has been in evidence for many decades it has been far greater towards the end of the 20th century and into the new millennium.

On the other hand, the average rate of ice loss from the Greenland ice sheet and the Antarctic ice sheet has increased substantially in the 21st century compared with the 20th century end. For Greenland, the increase in the rate of ice loss per year was fivefold more during 2002-11 compared with that during 1992-2001. Similarly, for the Antarctic ice sheet, the increase in the rate per year was nearly four times during 2002-11 compared with 1992-2001. These estimates are very likely , according to the report. While the Arctic sea ice extent—the latitudinal ocean area that is covered by ice at any given time and which is maximum in late winter/early spring and minimum in late summer/early fall—has decreased over the period, 1979-2012, assessed by the report, the Antarctic sea ice extent has increased during the same period. It is very likely , says the report, that the Arctic decrease was in the range 3.5-4.1 per cent of its area (0.45-0.51 million km) per decade. The rate of decrease in the summer was, in fact, greater at 9.4-13.1 per cent of its area.

The spatial extent of Arctic sea ice has retreated every season since 1979, the report says with high confidence . The report also notes with medium confidence that, over the past three decades, there was an unprecedented decrease in Arctic summer sea ice and that sea surface temperatures (SSTs) were anomalously high in the last 1,450 years. Multiple lines of evidence have led to this conclusion. Interestingly, however, the annual mean Antarctic sea ice extent increased at a rate of 1.2-1.8 per cent (0.13-0.2 million km) per decade during 1979-2012. The reason for this is not yet well understood.

The report concludes with very high confidence that the extent of northern hemisphere snow cover has decreased since the mid-20th century, and during 1967-2012, while it decreased at the rate of 1.6 per cent per decade in March and April, in summer (June) it decreased by a high 11.7 per cent.

Rise in mean sea level As a consequence of the glacial mass loss and ocean thermal expansion due to warming, the global MSL has increased significantly. According to the report’s high confidence assessment, the rate of sea level rise since the mid-19th century has been greater than the mean rate during the previous two millennia. Over the period 1901-2010, the global MSL rose by 0.19 m, with a tight 90 per cent uncertainty interval of 0.17-0.21 m. It is likely , says the report, that the rate of global MSL rise has continued to increase since the early 20th century. The very likely estimates of rate of MSL rise during different data set periods indicate that the rate of increase has been higher during the latter period. It was 1.9 millimetre/year during 1901-2010 and 2 mm/yr and 3.2 mm/yr respectively during the two periods of 1971-2010 and 1993-2010. Glacial mass loss and thermal expansion together account for about 75 per cent of this rise, says the report with high confidence .

‘Radiative forcing’ Natural and anthropogenic substances and processes that alter the earth’s energy budget are the drivers of climate change. Their influence is quantified by the parameter “radiative forcing” (RF), expressed in watts per square metre (W/m). It is the difference between the radiative energy received by the earth and that which is radiated back into space. Any change in these drivers will correspondingly change the net energy flux. A positive RF leads to surface warming and negative RF results in surface cooling, equivalently positive and negative global warming potential (GWP) respectively. In the context of the AR5, RFs for the different drivers have been calculated on the basis of changes in the drivers in 2011 relative to 1750 (pre-industrialisation period).

Since the total RF is positive, it has resulted in the uptake of energy by the climate system, and the largest contribution to total RF is caused by the increase in atmospheric CO since 1750. The total anthropogenic RF for 2011 relative to 1750 is 2.29 W/m and it has increased more rapidly since 1970 than during earlier decades, the report points out. The best estimate for total anthropogenic RF for 2011 is 43 per cent higher than what was reported in the AR4 for 2005. This, says the report, is because of a combination of a continued growth of most GHG concentrations and improved estimates of the net cooling effect of aerosols.

The RF from emissions of well-mixed GHGs—such as CO, methane (CH), nitrous oxide (NO) and halocarbons, which have lifetimes long enough to be relatively homogeneously mixed in the troposphere—for 2011 is 3.0 W/m to which the contribution due to emission of CO alone is 1.68 W/m. If you included other carbon-containing gases, which also contribute to increase in CO concentrations, the RF of CO is 1.83 W/m. While RF due to CH concentration itself has not changed from the AR4’s best estimate of 0.48 W/m, its effective RF has increased in 2011 to 0.97 owing to indirect changes in ozone and stratospheric water vapour concentrations caused by CH emissions.

While the depletion of the ozone layer itself has a negative RF (net cooling), the halocarbons that induce ozone depletion have a positive RF. The latter has now outweighed the former resulting in a net positive RF. The positive RF from all halocarbons is similar to what was reported in the AR4, according to the AR5 report. This is because, while the phaseout of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) has led to reduced RF from them, their substitutes, hydrofluorocarbons (HFCs), have much higher RFs or GWPs, resulting in no effective reduction in RF.

With United States President Barack Obama succeeding in finalising an agreement at the recently concluded G20 meeting, a switchover to new low GWP HFC substitutes may now become high on the global climate agenda. Anthropogenic RF contributions due to emissions of short-lived gases, too, have been estimated by the report. While carbon monoxide (CO) is assessed with virtual certainty to have induced a positive RF, oxides of nitrogen (NO) are likely to have induced a net negative RF ( Frontline , April 22, 2011).

The total RF contribution due to the effect of aerosols (fine solid particles) in the atmosphere, which includes cloud adjustments due to aerosols, has been estimated by the report with medium confidence . The resultant RF due to the negative forcings of most aerosols and a positive contribution of black carbon (soot) absorption of solar radiation has been estimated to be a net negative value of −0.9 W/m. “There is high confidence ,” says the report, “that aerosols and their interaction with clouds have offset a substantial portion of global mean forcing from well-mixed GHGs.” However, the report adds that the aerosol effect continues to have the largest uncertainty in the total RF estimate.

The atmospheric concentrations of CO, CH and NO are now the highest recorded in ice cores in the last 800,000 years, says the report. With very high confidence , the report has estimated that the rates of increase in the atmospheric concentrations of these GHGs are unprecedented in the past 22,000 years. Quantitatively, in 2011, the concentrations of CO, CH and NO were 391 parts per million, 1,803 parts per billion and 324 ppb respectively and these exceeded the pre-industrialisation levels (before 1750) by 40 per cent, 150 per cent and 20 per cent respectively. The increase in CO has largely been due to human activities, chiefly from fossil fuel emissions and land use changes. The oceans have absorbed 30 per cent of this anthropogenic CO, leading to their acidification.

Annual CO emissions from “ fossil fuel burning and cement production ”, averaged over 2002-11, were 8.3 gigatonnes of carbon (GtC) per year and were 9.5 GtC/yr in 2011 alone, 54 per cent above the 1990 level (1 GtC corresponds to 3.67 GtCO). Annual net emissions from land use changes were 0.9 GtC/yr on the average during 2002-11. The Kyoto Protocol targets were to stabilise global emissions to 1990 levels and to bring emissions of industrialised countries down to below 1990 levels by 2020. The first commitment period of the protocol ended in 2012 and the second commitment period was approved in the 2012 Conference of Parties of the United Nations Framework Convention on Climate Change but is yet to enter into legal force.

(It is not quite clear why the SPM document has singled out cement production by using the curious phrase “CO emissions from fossil fuel combustion and cement production”, giving the impression that cement production is the biggest CO-emitting sector. This has triggered protests from the cement industry worldwide. Of course, the cement industry is a major contributor to global CO emissions, accounting for about 5 per cent, but it contributes far less than the energy and transport sectors. Perhaps, it was singled out because nearly half of the CO emission in the cement industry is direct, as a result of the calcination process.)

From 1750 to 2011, fossil fuel combustion and other major CO-producing industrial activities have released 365 GtC into the atmosphere, while deforestation and other anthropogenic land use changes are estimated to have released 180 GtC, resulting in cumulative anthropogenic emissions of 545 GtC. While the 90 per cent uncertainty window for the latter is quite wide, that of the former is 335-365 GtC, which is pretty narrow. Of these cumulative emissions, 240 GtC have accumulated in the atmosphere, 155 GtC has been locked in the oceans, and 150 GtC has accumulated in terrestrial ecosystem sinks.

Continued emissions of GHGs will result in further warming and changes in all components of the climate system. Limiting climate change, notes the SPM, will require substantial and sustained reductions of GHG emissions. A hierarchy of models, from simple climate models to highly complex ones and Earth System Models (ESMs) are used to make projections of climate system changes. These models use a set of scenarios for anthropogenic forcings for simulation studies. (ESMs describe the processes within and between the atmosphere, the ocean, the cryosphere, and the terrestrial and marine biosphere completely through a system of equations.)

The AR5 has used a new set of scenarios called Representative Concentration Pathways (RCPs)—which include the full suite of GHGs and aerosols and chemically active gases as well as land use but exclude natural drivers such as solar or volcanic forcings or natural emissions—leading roughly to a particular value of the total RF in 2100 relative to 1750. The AR5 uses four RCPs—RCP2.6, RCP4.5, RCP6.0 and RCP8.5—with the total RF attaining the values of 2.6 W/m, 4.5 W/m, 6.0 W/m and 8.5 W/m respectively in 2100.

These four scenarios include one mitigation scenario (RCP2.6), two stabilisation scenarios (RCP4.5 and RCP6.0) and one with very high GHG emissions (RCP8.5). These can thus be used to represent different 21st century climate policies compared with the no-climate policy emission pathway scenarios used in the AR3 and the AR4. While these scenarios do include a range of total RF values, these do not cover the entire possible range of emissions used in climate studies, particularly for aerosols, the report emphasises.

Using the projection methodology with the new RCP scenarios, the report has drawn the following conclusions about the future surface temperature. Global surface temperature for the 21st century end is likely to exceed 1.5 °C relative to 1850 to 1900 for all scenarios except RCP2.6, which has the lowest RF. According to the report, 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. For all scenarios except for RCP2.6, warming will continue beyond 2100. It is unlikely that warming will exceed 4 °C for all scenarios and is about as likely as not to exceed for RCP8.5.

Impact on monsoon Significant among the projections made in the report pertaining to changes in the atmospheric water cycle is that globally it is likely that the area covered by monsoon systems will increase over the 21st century. Although monsoon winds are likely to weaken, increase in atmospheric moisture will likely lead to intensification of monsoon precipitation. The monsoon season could get longer in many regions, with onset dates likely to be earlier or not change much and withdrawal dates likely to be delayed. As the mean surface temperature increases, extreme precipitation events will very likely become more intense and more frequent over most of the mid-latitude land masses and wet tropical regions, says the report.

The El Nino-Southern Oscillation (ENSO) will continue to be the dominant mode of inter-annual variability in the tropical Pacific, with global effects in the 21st century, says the report with high confidence . ENSO-related precipitation variability on regional scales is likely to intensify owing to increased atmospheric moisture content. The report also makes similar projections with regard to other components of the climate system using the RCP scenarios. On the basis of ESM simulations, the report points out that climate change through to 2100 will affect carbon cycle processes for all scenarios in a way that will aggravate the CO increase in the atmosphere.

Limiting the warming caused by anthropogenic CO emissions alone to less than 2 °C with a probability of >33 per cent, >50 per cent and >66 per cent since the period 1861-80 will respectively require cumulative emissions from all anthropogenic sources to stay below 1,560 GtC, 1,210 GtC and 1,000 GtC since that period. However, emissions until 2011 have already touched 531 GtC. “A large fraction of anthropogenic climate change resulting from CO emissions is irreversible on a multi-century to millennial time scale, except in the case of a large net removal of CO from the atmosphere over a sustained period,” says the report ominously. “Surface temperatures,” it adds, “will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO emissions…. Depending on the scenario, about 15-40 per cent of emitted CO will remain in the atmosphere longer than 1,000 years.”