Climate Change: 2013 IPCC Update

4. How do we study the climate system?

• 4.1 Models, observations and validation
• 4.2 Why have global temperature increases slowed down in the past 15 years?
• 4.3 How do projections from previous assessments compare with the present observations?
• 4.4 How have the oceans changed?
• 4.5 Snow and Ice
• 4.6 What are the main climate changes that could be irreversible?

4.1 Models, observations and validation

Understanding of the climate system results from combining observations, theoretical studies of mechanisms and of feedback processes, and model simulations. Compared to the 4^th assessment report published in 2007, more detailed observations and improved climate models now enable the attribution of the observed climate changes to human influences in more components of the climate system. The consistency of observed and modelled changes across the climate system, including regional temperatures, the water cycle, global energy budget, cryosphere and oceans (including ocean acidification), reinforce that global climate changes result primarily from increase in atmospheric greenhouse gas concentrations of anthropogenic origin.

The warming of the earth is caused by an imbalance between the positive radiative forcing that is making the planet warmer, and the dissipation of energy toward space. The warmer the earth gets, the more energy is lost in space. If the radiative forcing was at a constant level, eventually a new balance would be reached, and no further warming would occur. However, the forcing is increasing, and the energy content of the climate system is increasing. It takes large amounts of energy to warm the oceans, and the observed change in ocean temperatures provides strong evidence of a changing climate. The quantification of the various elements in the Earth’s energy budget and the verification that these terms balance over recent decades provides strong evidence that our understanding of anthropogenic climate change is correct.

Several recent advances have allowed a more robust quantification of human influence on surface temperature changes. The currently observed temperature anomalies relative to the early 20^th century fall well outside of the range of simulations that consider only natural climate forcing. It is extremely likely that human activities caused more than half of the observed increase in global average surface temperature from 1951 to 2010. This assessment is supported by robust evidence from multiple studies using different methods. Greenhouse gases contributed to a global mean surface warming likely to be between 0.5°C and 1.3°C over the period between 1951 and 2010, with the contributions

• from other anthropogenic forcings likely to be comprised between –0.6°C and 0.1°C and
• from natural forcings likely to be comprised between –0.1°C and 0.1°C

Together, these assessed contributions are consistent with the observed warming of approximately 0.6°C over this period.

Solar forcing is likely to have had in influence on the global temperature over the 1951–2010 period but it has increased much less than forcing from greenhouse gas emissions. More...

4.2 Why have global temperature increases slowed down in the past 15 years?

The observed global-mean surface temperature (GMST) has shown a much smaller increase over the past 15 years than over the past 30 to 60 years. The warming over this period is estimated to have been around one-third to one-half of the trend over 1951–2012. Changes in radiative forcing between decades show that for the period 1998-2011 forcing was indeed two thirds of what is had been for the 1984-1998 period. Changes in natural forcings were also observed , from volcanic eruptions for instance, and from solar forcing that went from a maximum in 2000 to a minimum in 2009.

The uncertainties about the radiative forcings are relatively large, there doesn’t seem to be a missing factor to explain the variability. Even with this pause in the global temperature trend, the decade of the 2000s has been the warmest in the instrumental record. Nevertheless, the occurrence of the hiatus in GMST trend during the past 15 years raises related questions:

1. What has caused it?
2. Are climate models able to simulate it?

Such fifteen-year-long hiatus periods are common in both the observed and historical temperature records. However, model simulations presently do not fit these recent observed records, which could be caused by an error in the response of the models, by a forcing – such as water vapour – that is not yet taken into account in the models, or by a variation in the climate that falls outside the present climate models limits. A period of 5 years is indeed too short to observe the effect of long-term trends that can be also affected by oceanic currents variations like those of El Niño or of the Interdecadal Pacific Oscillation.

Globally, it is very likely that over that period the climate system has continued to accumulate energy, for instance in the form of increasing ocean heat, although, since some data show a slowing, and some do not, whether there has been a slowdown in that recent period remains unclear. More...

4.3 How do projections from previous assessments compare with the present observations?

Verification of projections is arguably the most convincing way of establishing the credibility of the models used in climate change science. Results of projected changes in CO₂, global mean surface temperature and global mean sea level from previous IPCC assessment reports can be compared with the best available present recorded observations.

Comparison of modeled and measured atmospheric CO₂ evolution

The observed changes in atmospheric concentrations of CO₂ fits within the range of the scenarios that have been used in the four previous assessment reports.

Comparison of modeled and measured Global Mean Temperature evolution

Overall the observed temperature record lies within the total range of uncertainty of the models. The global mean surface temperature has been higher than the 1961-1990 average by at least 0.25°C since 2001. The eruption of Mount Pinatubo in 1991 lead to a brief period of relative global mean cooling during the early 1990’s that was not taken into account in the models used in the first, second and third assessment reports. More recent models, however, did include the impact from volcanoes emissions and did simulate successfully the cooling associated to the Pinatubo eruption. From 1998–2012, the observational estimates have largely been on the low end of the range given by the scenarios of the previous reports.

Comparison of modeled and measured Global Mean Sea Level

Based on both tide gauge and satellite altimetry data, relative to 1990, the global mean sea level has continued to rise. The observed estimates lie within the envelope of all the projections. More...

4.4 How have the oceans changed?

Various oceanic parameters have been monitored and modelled in terms of their response to climate change: temperature, salinity, oxygen content and acidity.

• Temperature The observed upper-ocean warming observed during the late 20^th and early 21^st centuries and its causes have been assessed more completely since the previous report using updated observations and more model simulations. It is very likely that anthropogenic radiative forcing have made a substantial contribution to the upper ocean warming (the upper 700m layer) that has been observed since the 1970s. This warming has in turn contributed to a global sea level rise through thermal expansion. It is estimated that more than 90% of the energy that has been added to the climate system has been absorbed by the oceans.
• Salinity On a global scale, surface and subsurface salinity changes (1955–2004) over the upper 250m of the water column are very unlikely to be explained by natural variability and the observed salinity changes match the modelled distribution of forced changes produced by greenhouse gases and tropospheric aerosols emissions.
• Oxygen is an important physical and biological tracer in the ocean. Global analyses of oxygen data from the 1960’s to 1990’s extend the spatial coverage from local to global scales and have been combined with attribution studies for a limited range of earth system models. There is presently medium confidence that the observed global pattern of decrease in dissolved oxygen in the oceans can be attributed in part to human influences.
• Acidification The observations show distinct trends for ocean acidification (which is observed to be between –0.0014 and –0.0024 pH units per year). It is very likely that oceanic uptake of anthropogenic carbon dioxide has resulted in the acidification of surface waters.

More...

4.5 Snow and Ice

The reductions in Arctic sea ice extent and Northern hemisphere snow cover extent and widespread glacier reduction (retreat)- and increased surface melt of Greenland are all evidence of overall changes in snow and ice linked to increased radiative forcings of anthropogenic origin.

It is likely that anthropogenic forcing has contributed to surface melting of the Greenland ice sheet since 1990. By contrast, the understanding of the Antarctic system is more fragmentary, and it is premature to suggest that anthropogenic forcings are at play in this case Antarctic sea ice extent has increased since 1979, but there are a lot of factors that make the modelling difficult and the understanding incomplete. Estimates of the mass balance of the Antarctic ice sheet since 2000 show that the greatest losses are at the edges, likely because of oceanic warming.

For glaciers, there is high confidence that a substantial part of their mass loss is likely due to human influence. For snow cover, it is also likely that there has been an influence of human activity to the observed reductions in the Northern hemisphere since 1970. More...

4.6 What are the main climate changes that could be irreversible?

The rate and magnitude of global climate change is determined by radiative forcing, climate feedbacks and the storage of energy by the climate system. For some elements of the climate system, there is a point where an abrupt change might happen once a certain threshold is reached. These abrupt changes can be irreversible transitions to different states of the climate system – meaning here that it takes a lot longer for the system to recover than it takes to shift to the new state. More...

4.6.1 Recent climate model confirmed that changes in the Atlantic meridional overturning circulation (AMOC) could produce abrupt climate changes at global scale and on the climate of Europe and North America, with magnitude and pattern resembling the rapid warming and cooling pulses that took place during the last glaciation. However, while it is very likely that the AMOC will weaken over the 21^st century, it is very unlikely that it will undergo an abrupt transition or collapse over that period. More...

4.6.2 In a warming climate, the thawing of permafrost could lead to the release of carbon accumulated in frozen soils, leading to an increase of atmospheric CO₂ and methane concentrations. It is projected that present-day permafrost will become a net emitter of carbon during the 21^st century. However, it is difficult to quantify the impact of permafrost thaw due to the lack of understanding of the soil processes during and after permafrost thaw. More...

4.6.3 Since the growth of the ice sheets is a very slow process, any increase in the loss of ice, either through melt or ice outflow would be irreversible as defined here. At present, on the surface of both the Greenland and Antarctic ice sheets snowfall exceeds melting, but both are losing mass because of ice outflow into the sea. While there are still a lot of uncertainties, it is estimated that beyond a level of global temperature rise somewhere between 2°C and 4°C of), the Greenland Ice Sheet would melt almost entirely, over several centuries, causing a global mean sea level rise of approximately 7 m. More...