by Judith Curry
The recent pause in global surface temperature rise does not, in itself, materially alter the risks of substantial warming of the Earth by the end of this century. – UK Met Office
Bishop Hill points to three papers released today by the Met Office on the topic of the pause:
- Paper 1: Observing changes in the climate system (PDF, 2 MB)
- Paper 2: Recent pause in global warming: What are the potential causes? (PDF, 1 MB)
- Paper 3: Implications for projections (PDF, 664 kB)
Below are some excerpts and my comments:
Paper 1: Observing changes in the climate system
Paper 1 provides a good observational summary of observations of a range of climate variables over the past 2-5 decades. In looking at these plots, I was struck by the disagreement among the different measurements/estimates of the same variable, notably humidity/water vapor and cloudiness.
A 15+ yr pause seems to be reflected in the following variables:
- surface temperature
- lower tropospheric temperature
- lower stratospheric temperature
- NH hemisphere snow extent
- total column water vapor
From the conclusions:
It has shown that a wide range of observed climate indicators continue to show changes that are consistent with a globally warming world, and our understanding of how the climate system works.
Paper 2: Recent pause in global warming
Punchline from the Executive Summary:
It is not possible to explain the recent lack of surface warming solely by reductions in the total energy received by the planet, i.e. the balance between the total solar energy entering the system and the thermal energy leaving it. Observations of ocean heat content and of sea-level rise suggest that the additional heat from the continued rise in atmospheric carbon dioxide concentrations has been absorbed in the ocean and has not been manifest as a rise in surface temperature. Changes in the exchange of heat between the upper and deep ocean appear to have caused at least part of the pause in surface warming, and observations suggest that the Pacific Ocean may play a key role.
There are potentially two distinct mechanisms to explain the recent pause; the first involves changes to the total energy received by the planet (radiative forcing), and the second involves the low frequency variability of the oceans and the way in which the oceans take up heat and store it below the surface, potentially into the deeper ocean. It is possible that a pause in surface warming could result from both mechanisms acting together.
From the section Is the current pause in global warming unusual?:
It is clear that there have been other periods with little or no surface warming in the relatively recent past, a good example being the period between the 1940s and the 1970s (Figure 1). The trend in warming over that period is well understood, and linked to a substantial increase in the amount of aerosol in the atmosphere.
JC comment: The 1940s to 1970s pause is not well understood; even with over juicing the climate models with aerosols for this period, they still don’t reproduce the pause. The aerosol explanation doesn’t hold up. I suspect it has mostly the same cause as the current pause, associated with changes in the ocean circulation patterns, notably the PDO and AMO.
The start of the current pause is difficult to determine precisely. Although 1998 is often quoted as the start of the current pause, this was an exceptionally warm year because of the largest El Niño in the instrumental record. This was followed by a strong La Niña event and a fall in global surface temperature of around 0.2oC (Figure 1), equivalent in magnitude to the average decadal warming trend in recent decades. It is only really since 2000 that the rise in global surface temperatures has paused.
JC comment: Here we see some spin. If the pause starts in 2000, it is further away from the 10, 15, 17, 20 yr thresholds variously argued for the length of a pause associated with natural internal variability. However from a climate dynamics perspective, I do agree that the shift began circa 2001 with changes to the ocean circulation.
New unpublished climate model results are cited:
Secondly, the results show that a pause of 10 years’ duration is likely to occur due to internal fluctuations about twice every century. Thirdly, the results also show that beyond periods of 20 years and longer, a pause of that duration occurring from natural, internal variability in the absence of other changes in external forcing appears to be unlikely.
More research remains to be done to investigate to what degree the current pause in global surface warming is unusual.
JC comment: this is why we need reliable paleclimate reconstructions with subdecadal resolution.
The most interesting text is in the section What do we know about changes in the ocean heat budget? Excerpts:
As Figure 4 shows, the North Atlantic warmed rapidly during the latter half of the 20th century and this may well have contributed to the rate of global surface temperature rise in that period. It has been estimated that variations in the AMO can give fluctuations of about 0.1°C in global temperature (Knight et al 2005). Since then the AMO has remained fairly constant, so it is unlikely that fluctuations in North Atlantic surface temperatures have contributed to the recent pause in global surface warming, although the deeper Atlantic may be storing some of the heat.
JC comment: I have seen higher estimates than 0.1C of the AMO contribution to global temperature
The timeseries of the PDO index (Figure 5, lower panel) shows multi-decadal variations in the phase of the PDO. The transition from the negative to the positive phase in the late 1970s has been widely documented, and is often referred to as the ‘1976 climate shift’ (e.g. Miller et al. 1994, Trenberth and Hurrell 1994). The PDO index has shifted back to its negative phase since the turn of the Millennium and the question is whether this has been a contributor to the recent pause in global mean surface warming.
Measurements of heat content in the deeper ocean are much sparser and hence less certain. Using a combination of satellite and ocean measurements down to 1800m, Loeb et al (2012) estimate the Earth system has been accumulating energy at a rate of 0.5±0.4 Wm-2 from 2001 to 2010, similar to the 0.4Wm-2 from 2005 to 2010 down to around 1500m estimated from Argo floats alone (von Schuckman and Le Traon 2011). Reanalyses of ocean data give an average rate of warming from 2000 to 2010 of about 0.9Wm-2 averaged over the globe, with 30% of the increase occurring below 700m (Balmaseda et al. 2013). We conclude that the Earth system has continued to absorb a substantial amount of heat during the last 15 years, despite the pause in surface warming.
JC comment: Interesting that Balmaseda et al estimate is about a factor of 2 higher than data only estimates.
However, prior to 1965 and from 2000 to the present day, there are substantial differences between the net radiative flux and the upper ocean heat uptake (black dashed curve in Figure 9), implying heat taken up by other components of the climate system, most likely the ocean below 800m. It is notable that there is a pause in the global mean surface temperature rise during both periods, and that the PDO was also in a strong negative phase
JC comment: This provides a rationale for reconsidering the aerosol explanation for the the previous 1940s to 1970s pause.
Focussing on the recent period in more detail, the onset of the current pause coincides with a maximum in upper ocean heat uptake around 2002, and may reflect a recovery both from Mount Pinatubo and from the record 1997/98 El Niño. A recent study (Guemas et al, 2013) shows that, in 2002, the upper ocean below the mixed layer took up heat, while the mixed layer and sea surface temperature did not warm. The onset of the pause may therefore have been caused by ocean processes, predominantly in the tropical Pacific, in which energy trapped by greenhouse gases was buried below the surface of the ocean.
However, the continuation of the pause in global surface warming beyond 2004 coincides with a decline in upper ocean heat uptake (Figure 9). Previous minima in heat uptake are often associated with volcanic eruptions, but the decline in heat uptake after 2002 cannot be explained by a major volcanic eruption. Understanding the cause of this decline in upper ocean heat content is therefore crucial for explaining the continuation of the pause in surface warming. As already noted the monitoring of upper ocean heat content has changed substantially over the last decade with the rapid increase in deployment and hence global coverage of floats.
JC comment: again, this is evidence of a climate shift circa 2001
If, however, the observations are robust, then the maximum in upper ocean heat uptake in the early part of this decade and the subsequent minimum in upper ocean heat uptake cannot be explained by changes in net radiative fluxes, as shown by large residuals in Figure 10. This suggests that the pause in global surface warming is unlikely to have been caused solely by systematic changes in the top of the atmosphere radiation associated with solar variability and minor volcanic eruptions, anthropogenic aerosol emissions, or changes in stratospheric water vapour as suggested in other studies.
JC comment: this acknowledgment that not all climate change is forced is important; unfortunately they only seem to apply it to the recent pause and not the warming in the 1980’s and 1990’s.
Although this analysis suggests that exchanges of energy between the upper and deep ocean, calculated here as a residual (see black dashed line in Figure 10), may be of a similar magnitude to upper ocean heat uptake and net radiative forcing, we cannot show definitively that this has been the dominant factor in the recent pause in global surface warming. The fact is that uncertainties in estimating upper ocean heat content from the current monitoring network, along with uncertainties in observing the net radiation budget already discussed, mean that the residual calculation of deep ocean heat flux has to be treated with limited confidence.
In addition, direct measurements of the exchange of heat between the upper and deep ocean do not exist because the present ocean observing network does not sample the ocean below 2000m adequately. Even if it did, the potential changes in temperature could be very small, remembering that the energy imbalances involved are less than 1Wm-2 (Figure 10) and therefore potentially not detectable as temperature changes. However, some ocean analyses (Balmaseda et al, 2013; Levitus et al, 2012) show a continued uptake of heat by the deeper ocean throughout the period, consistent with our conclusion that changes in TOA R may not play a leading role.
JC comment: This acknowledges the deficiencies and uncertainties in magnitude anyways of the recent deep ocean heat uptake.
From the conclusions:
What can we conclude from all this? First, periods of slowing down and pauses in surface warming are not unusual in the instrumental temperature record.Second, climate model simulations suggest that we can expect such a period of a decade or more to occur at least twice per century, due to internal variability alone. Third, recent research suggests that ocean heat re-arrangements, with a contribution from changes in top of the atmosphere radiation, could be important for explaining the recent pause in global surface warming.
JC comment: They seem to think that this pause is not unusual or even unexpected. 16 years of ‘pause’ and counting brings us very close to falling completely outside of the large envelope of climate model simulations. You can see why they want to redefine the pause to begin in 2000
The scientific questions posed by the current pause in global surface warming require us to understand in much greater detail the flows of energy into, out of, and around the Earth system. Current observations are not detailed enough or of long enough duration to provide definitive answers on the causes of the recent pause, and therefore do not enable us to close the Earth’s energy budget. These are major scientific challenges that the research community is actively pursuing, drawing on exploration and experimentation using a combination of theory, models and observations.
JC comment: Yes, the science is far from settled. We cannot close the Earth’s energy budget, and our models don’t adequately simulate multidecadal ocean variability.
Part 3: What are the implications of the pause for projections of future warming?
From the Concluding remarks:
Despite the fact that the first decade of the 21st century has been a period during which there was very little global mean surface temperature rise, the range of TCR estimates from the CMIP5 models lies within the TCR derived from observations, including this period. Indeed it can be shown that even the projections from much earlier models encompass the subsequent surface temperature observations, including the most recent decade. Therefore the physical basis of climate models and the projections they produce have not been invalidated by the recent pause in global surface temperature rise.
When projections from the newer CMIP5 models are combined with observations, and specifically including the surface temperatures from the last 10 years, the upper bound of projections of warming are slightly reduced, but the lower bound is largely unchanged. More importantly, the most likely warming is reduced by only 10%, indicating that the warming that we might previously have expected by 2050 would be delayed by only a few years.
Observational constraints on the ECS are more problematic because of uncertainties in energy storage in the Earth system. Again the models continue to provide a consistent range of values for the ECS, lying within the uncertainty range of the observationally-based estimates.
In conclusion, the recent pause in global surface temperature rise does not invalidate climate models or their estimates of climate sensitivity. It does however raise some important questions about how well we understand and observe the energy budget of the climate system, particularly the important role of the oceans in taking up and redistributing heat, as highlighted in the second report. In particular, this report emphasises that the recent pause in global surface warming does not, in itself, materially alter the risks of substantial warming of the Earth by the end of this century.
JC comment: They dismiss all of the recent empirical estimates of low climate sensitivity, and continue to think climate models are adequate.
There is some good material in these reports. But they draw some conclusions that seem to me to be unwarranted, and further miss an opportunity to ‘cover their backs’ if the pause does indeed continue for another 2-3 decades by acknowledging the importance of multidecadal natural internal variability in explaining the 20th century record.
They seem to obliquely admit the inadequacy of climate models by saying that they have not been falsified by the recent pause. Well, even if they have not been falsified, the climate models are not looking very useful at the moment, and climate model-derived values of climate sensitivity are seeming increasingly unconvincing.
Update: Walter Mead sums it up this way:
There are innumerable variables in the climate system that could be responsible for the warming slowdown. These scientists have identified some of the likeliest culprits, but one professor admitted that they “don’t fully understand the relative importance of these different factors.”
That’s a big problem, considering most green legislation aimed at reducing emissions calls for measures to prevent very specific degrees of warming. This recent warming plateau is exposing our limited understanding of climate, and it’s effectively killing the rationale for green policies that limit growth and, at the most basic level, try to force people to do things they would rather not do. The biggest cause of climate skepticism isn’t evil oil companies and campaigns of disinformation; it is the inability of greens to refrain from overstating their case and insisting dogmatically and self righteously on more certainty than the frustrating facts can give.