by Judith Curry
“For the next two decades a warming of about 0.2C per decade is expected for a range of emission scenarios.” – IPCC AR4
In response to my post on Steven Hayward, Hayward is being criticized on twitter for this statement made in context of discussing the temperature plateau:
The basic theory says we’re supposed to continue warming at about 0.2 degrees Celsius per decade.
Seems pretty consistent with what the AR4 SPM said.
Lets dig deeper to see what the IPCC AR4 actually said. From the SPM:
For the next two decades, a warming of about 0.2°C per decade is projected for a range of SRES emission scenarios. Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected.
Since IPCC’s first report in 1990, assessed projections have suggested global average temperature increases between about 0.15°C and 0.3°C per decade for 1990 to 2005. This can now be compared with observed values of about 0.2°C per decade, strengthening confidence in near-term projections.
Model experiments show that even if all radiative forcing agents were held constant at year 2000 levels, a further warming trend would occur in the next two decades at a rate of about 0.1°C per decade, due mainly to the slow response of the oceans. About twice as much warming (0.2°C per decade) would be expected if emissions are within the range of the SRES scenarios. Best-estimate projections from models indicate that decadal average warming over each inhabited continent by 2030 is insensitive to the choice among SRES scenarios and is very likely to be at least twice as large as the corresponding model-estimated natural variability during the 20th century.
Chapter 10 Global Climate Projetions provides details on the projections. From the Executive Summary:
There is close agreement of globally averaged SAT (surface air temperature) multi-model mean warming for the early 21st century for concentrations derived from the three non-mitigated IPCC Special Report on Emission Scenarios (SRES: B1, A1B and A2) scenarios (including only anthropogenic forcing) run by the AOGCMs (warming averaged for 2011 to 2030 compared to 1980 to 1999 is between +0.64°C and +0.69°C, with a range of only 0.05°C). Thus, this warming rate is affected little by different scenario assumptions or different model sensitivities, and is consistent with that observed for the past few decades . Possible future variations in natural forcings (e.g., a large volcanic eruption) could change those values somewhat, but about half of the early 21st-century warming is committed in the sense that it would occur even if atmospheric concentrations were held fixed at year 2000 values.
So, if we translate the bolded numbers into decadal averages (divide by 3.1 decades), we get a range of 0.206 to 0.226C.
From section 10.3.1:
Internal variability in the model response is reduced by averaging over 20-year time periods. This span is shorter than the traditional 30-year climatological period, in recognition of the transient nature of the simulations, and of the larger size of the ensemble. The close agreement of warming for the early century, with a range of only 0.05°C among the SRES cases, shows that no matter which of these non-mitigation scenarios is followed, the warming is similar on the time scale of the next decade or two. Note that the precision given here is only relevant for comparison between these means. As evident in Figure 10.4 and discussed in Section 10.5, uncertainties in the projections are larger. It is also worth noting that half of the early-century climate change arises from warming that is already committed to under constant composition (0.37°C for the early century).
From this, I infer that the AR4 states that it expects warming in the early decades of the 21st century to be at least 0.2C per decade, when averaged over two decades.
While section 10.5 discusses climate model uncertainties in a general sense (e.g. sensitivity), there is no uncertainty estimate for the near term projections. You can see some spread among the models in Figure 10.4, but this does not enter explicitly into the discussion of the projections. There is nowhere in the AR4 that I can find a confidence level or an uncertainty estimate for the 0.2C/decade.
From the AR5 SPM:
The global mean surface temperature change for the period 2016–2035 relative to 1986–2005 will likely be in the range of 0.3°C to 0.7°C (medium confidence). This assessment is based on multiple lines of evidence and assumes there will be no major volcanic eruptions or secular changes in total solar irradiance.
To make this comparable to the AR4 numbers, 0.3 to 0.7C over a period of 30 years translates to 0.1 to 0.23C/decade. The AR5 places medium confidence on this number. The AR5 clearly did a better job than AR4, by providing a confidence level and a much larger range of uncertainty. MOST SIGNIFICANTLY, Ch 11 states:
However, the implied rates of warming over the period from 1986–2005 to 2016–2035 are lower [than the model simulations] as a result of the hiatus: 0.10°C–0.23°C per decade, suggesting the AR4 assessment was near the upper end of current expectations for this specific time interval.
Note: this is discussed more fully in the previous CE post IPCC AR5 weakens the case for AGW. In terms of actual model projections for the period 2012-2035, the CMIP5 5-95% trend range is 0.11°C–0.41°C per decade.
Earlier IPCC Reports
Some insights can be gained from earlier IPCC reports into how the AR4 became so confident of the 0.2C/decade warming in the early 21st century.
The First Assessment Report (FAR, 1990) stated:
… our best estimate of global mean warming of 1.8 C by 2030 (relative to pre-industrial).
From a subsection Other factors which could influence future climate:
Because of long-period couplings between different components of the climate system, for example between ocean and atmosphere, the Earth’s climate would still vary without being perturbed by any external influences. This natural variability could act to add to, or subtract from, any human-made warming, on a century time-scale this would be less than changes expected from greenhouse gas increases
From a subsection How much confidence do we have in our predictions?
Thirdly, climate models are only as good as our understanding of the processes which they describe, and this is far from perfect The ranges in the climate predictions given above reflect the uncertainties due to model imperfections, the largest of these is cloud feedback (those factors affecting the cloud amount and distribution and the interaction of clouds with solar and terrestrial radiation), which leads to a factor of two uncertainty in the size of the warming Others arise from the transfer of energy between the atmosphere and ocean, the atmosphere and land surfaces, and between the upper and deep layers of the ocean. The treatment of sea-ice and convection in the models is also crude Nevertheless, for reasons given in the box overleaf, we have substantial confidence that models can predict at least the broad scale features of climate change.
Further excerpts from the SPM:
Since the end of the last ice age, about 10,000 years ago, global surface temperatures have probably fluctuated by little more than 1°C . Some fluctuations have lasted several centuries, including the Little Ice Age which ended in the nineteenth century and which appears to have been global in extent.
The size of the warming over the last centuiy is bioadly consistent with the predictions ol climate models but is also ol the same magnitude as natural climate variability. If the sole cause ol the observed wanning were the human made greenhouse effect, then the implied climate sensitivity would be near the lower end of the range from the models. The observed increase could be largely due to natural variability, alternatively this variability and other man-made factors could have offset a still larger man-made greenhouse warming The unequivocal detection of the enhanced greenhouse effect from observations is not likely lor a decade or more, when the committment to future climate change will then be considerably larger than it is today.
The Second Assessment Report (SAR, 1995):
The SAR did not focus on the period out to circa 2030 (focusing on the period out to 2100), but the Technical Summary includes the following text:
F.5 The possibility of surprises Unexpected external influences, such as volcanic eruptions, can lead to unexpected and relatively sudden shifts in the climatic state. Also, as the response of the climate system to various forcings can be non-linear, its response to gradual forcing changes may be quite irregular. Abrupt and significant changes in the atmospheric circulation involving the North Pacific which began about 1976 were described in IPCC (1990). A related example is the apparent fluctuation in the recent behaviour of ENSO, with warm conditions prevailing since 1989, a pattern which has been unusual compared to previous ENSO behaviour. Another example is the possibility that the West Antarctic ice sheet might “surge”, causing a rapid rise in sea level. The current lack of knowledge regarding the specific circumstances under which this might occur, either in total or in part, limits the ability to quantify this risk. Nonetheless, the likelihood of a major sea level rise by the year 2100 due to the collapse of the West Antarcfic ice sheet is considered low.
In the oceans the meridional overturning might weaken in a future climate. This overturning (the thermohaline circulation) is driven in part by deep convection in the northern North Atlantic Ocean and keeps the northern North Atlantic Ocean several degrees warmer than it would otherwise be. Both the study of palaeoclimate from sediment records and ice cores and modelling studies with coupled climate models and ocean GCMs can be interpreted to suggest that the ocean circulation has been very different in the past. Both in these observations and in the ocean models, transitions between different types of circulation seem to occur on a time-scale of a few decades, so relatively sudden changes in the regional (North Atlantic, Western Europe) climate could occur, presumably mainly in response to precipitation and runoff changes which alter the salinity, and thus the density, of the upper layers of the North Atlantic. Whether or not such a sudden change can actually be realised in response to global warming and how strong a perturbation is required to cause a transition between types of circulation are still the subject of much debate.
The Third Assessment Report (TAR, 2001), from the SPM:
On timescales of a few decades, the current observed rate of warming can be used to constrain the projected response to a given emissions scenario despite uncertainty in climate sensitivity. This approach suggests that anthropogenic warming is likely to lie in the range of 0.1 to 0.2°C per decade over the next few decades under the IS92a scenario.
The IPCC AR4 stands out, among the other four Reports, as providing a projection of 0.2C/per decade for the early 21st century, with a tiny uncertainty range. The projection is qualified only by the remark that 2 decades of averaging are needed to reduce internal variability in model response.
The IPCC AR4 was arguably the most important report politically, motivating many of the national responses to climate change that are now in place.
It was interesting going back through the previous IPCC reports – the FAR is far and away the best one. SAR isn’t too bad, although the ‘discernible’ piece that emerged from the meeting with the policy makers has raised substantial concerns. The TAR reads more like propaganda (with the hockey stick as its centerpiece). The main thing that comes across in the AR4 is hubris associated with the climate models. The AR5 is peculiar mainly in context of the disconnect between the confidence levels of the SPM and what is written in the main text of the Report.
So, what happened between the SAR and AR4? The curse of the hockey stick, whereby natural climate variability for the past 1000 years was alleged to be only a few tenths of a degree (goodbye MWP, LIA). This led to the belief that significant climate variability on timescales of more than a decade only arose from external forcing; a few blips from volcanoes and then anthropogenic impacts – and by inference, the climate models were able to simulate climate change. And what happened between AR4 and the AR5? Well, the hiatus.
And finally, take a look at this piece by Euan Mearns comparing the various projections of the different IPCC reports with observations. While the analysis is simple, it is straightforward and clearly written.