by Judith Curry
Some interesting new papers on the hiatus in global warming.
I have been expecting to start seeing papers on the ‘hiatus is over.’ Instead I am seeing papers on ‘the hiatus never happened.’ Here is a collection of new papers on the hiatus, ranging from sense to nonsense.
The UKMO has issued a good report Big Changes in the Climate System Underway? The report discusses the current situation (and near future possibilities) for El Nino, PDO, AMO. Re the AMO, which was the subject of a recent post:
Despite these signals it is not certain that there will be a shift towards cooler Atlantic conditions over the next few years. Temporary cooling has occurred in the past without leading to a sustained AMO shift. However, the current trends suggest that the chances of a shift in the next few years have increased.
The report has a good section on global temperatures, excerpts:
A better understanding of the cause of the global warming slowdown is needed in order to confidently predict its end. However, there are signs in the observations and near term climate predictions that are consistent with a resumption of warming.
Global mean surface temperature for 2015 so far is 0.38±0.14oC above the 1981-2010 average (0.68±0.14oC above the 1961-1990 average). If this continues, 2015 will likely be warmer than any other year in the observational record. Given that global average temperature responds to El Niño with a lag of a few months and that El Niño is predicted to develop further, it is reasonable to assume that both 2015 and 15 2016 will show similar warmth to the current value for 2015. If this were to happen then ten year global temperature trends would increase to a value of around 0.2C per decade by 2016, as often occurred in the late 20th century. Note however, that a large volcanic eruption or a sudden shift to a cool phase of the AMO would alter this and that trends over longer periods of 15 years would take longer to respond.
Norwegian climate model evaluation
A two part paper from a Norwegian team (both papers are open access):
Stephen Outten, Peter Thorne, Ingo Bethke, Øyvind Seland
The recent Intergovernmental Panel on Climate Change report, along with numerous studies since, has suggested that the apparent global warming hiatus results from some combination of natural variability and changes to external forcings. Herein the external forcings for greenhouse gases (GHGs), long-lived trace gases, volcanic and tropospheric aerosols, and solar irradiance have been replaced in the Norwegian Earth System Model using recent observational estimates. The potential impact of these alternative forcings, and by residual the internally generated variability, is examined through two 30-member ensembles covering the period 1980 to 2012. The Reference ensemble uses the Coupled Model Intercomparison Project phase 5 historical forcings extended with the Representative Concentration Pathway 8.5 (RCP8.5) scenario, while the Sensitivity ensemble uses the alternative forcings. Over the hiatus period defined herein as 1998–2012, all of the forcings show some change between the Sensitivity and Reference experiments and have a combined net forcing change of −0.03 W m−2. The GHG forcing is 0.012 W m−2 higher in the Sensitivity forcings. The alternative solar forcing differs from the Reference forcing by −0.08 W m−2, the same as the alternative volcanic forcing that was based on the latest estimates from NASA Goddard Institute for Space Studies. Anthropogenic aerosol emissions were replaced using the EU-EclipseV4a data set and produce a mean forcing change of 0.11 W m−2 over the period. Part 1 details the creation of the two 30-member ensembles and their characterization for parameters of particular relevance to the explanation of the hiatus. A detailed investigation of the two resulting ensembles global surface temperature behavior is given in Part 2, along with comparisons to observational data sets.
Peter Thorne, Stephen Outten, Ingo Bethke, Øyvind Seland
Abstract. To assess published hypotheses surrounding the recent slowdown in surface warming (hiatus), we compare five available global observational surface temperature estimates to two 30-member ensembles from the Norwegian Earth System Model (NorESM). Model ensembles are initialized in 1980 from the transient historical runs and driven with forcings used in the CMIP5 experiments and updated forcings based upon current observational understanding, described in Part 1. The ensembles’ surface temperature trends are statistically indistinguishable over 1998–2012 despite differences in the prescribed forcings. There is thus no evidence that forcing errors play a significant role in explaining the hiatus according to NorESM. The observations fall either toward the lower portion of the ensembles or, for some observational estimates and regions, outside. The exception is the Arctic where the observations fall toward the upper ensemble bounds. Observational data set choices can make a large difference to findings of consistency or otherwise. Those NorESM ensemble members that exhibit Nino3.4 Sea Surface Temperature (SST) trends similar to observed also exhibit comparable tropical and to some extent global mean trends, supporting a role for El Nino Southern Oscillation in explaining the hiatus. Several ensemble members capture the marked seasonality observed in Northern Hemisphere midlatitude trends, with cooling in the wintertime and warming in the remaining seasons. Overall, we find that we cannot falsify NorESM as being capable of explaining the observed hiatus behavior. Importantly, this is not equivalent to concluding NorESM could simultaneously capture all important facets of the hiatus. Similar experiments with further, distinct, Earth System Models are required to verify our findings.
JC comment: While there are no punchline take away messages from these two papers (well maybe the bolded statement above), I find them to be valuable contributions. The papers provide a very detailed evaluation of a single climate model in context of the hiatus, using alternative forcings and alternative observational data sets.
Cowtan et al.
A new paper from Cowtan et al:
Kevin Cowtan, Zeke Hausfather, Ed Hawkins, Peter Jacobs, Michael E. Mann, Sonya K. Miller, Byron A. Steinman, Martin B. Stolpe, Robert G. Way
Abstract. The level of agreement between climate model simulations and observed surface temperature change is a topic of scientific and policy concern. While the Earth system continues to accumulate energy due to anthropogenic and other radiative forcings, estimates of recent surface temperature evolution fall at the lower end of climate model projections. Global mean temperatures from climate model simulations are typically calculated using surface air temperatures, while the corresponding observations are based on a blend of air and sea surface temperatures. This work quantifies a systematic bias in modelobservation comparisons arising from differential warming rates between sea surface temperatures and surface air temperatures over oceans. A further bias arises from the treatment of temperatures in regions where the sea ice boundary has changed. Applying the methodology of the HadCRUT4 record to climate model temperature fields accounts for 38% of the discrepancy in trend between models and observations over the period 1975-2014.
Co author Ed Hawkins has a blog post on this: An apples-to-apples comparison of global temperatures. The key point here is this:
Usually, global temperatures from climate models use simulated near-surface air temperatures (SATs). However, observations use SATs measured over land, and sea-surface temperatures (SSTs) measured over the ocean and are not spatially complete. The comparison of the global averages of these two different quantities is therefore not like-with-like.
This new analysis gets closer to a like-with-like comparison by sampling the models with the same spatial coverage as the observations, and by using the simulated SSTs over the ocean, blended with the SATs over the land. Note that more background, along with the code & data is available for this study.
Because SSTs tend to warm slightly slower than SATs over the ocean, this reduces the warming trend in the simulated estimates of global temperature, bringing them closer to the observations over the past decade. This simple correction accounts for about one third of the difference between the observations and simulations (top panel).
JC comment: This paper makes the important point that blending surface air temperature over land with bulk ocean surface temperatures over ocean introduces some problems (well they aren’t the first ones to make this point). The key problem I have with their analysis is that they rely on global climate model simulations to sort out the differences between trends in surface air temperature and bulk surface temperature over the oceans. The odds of climate models doing this correctly are slim to none IMO. Note, this particular subject, of differences between bulk ocean surface and surface air temp was a focus of my research circa 1994-2002, here are a few relevant papers:
Clayson, C.A. and J.A. Curry, 1996: Determination of surface turbulent fluxes for TOGA COARE: Comparison of satellite retrievals and in situ measurements. J. Geophys. Res., 101, 28,503-28,513.
Curry, J.A., J.L. Schramm, A. Alam, R. Reeder, T.E. Arbetter, P. Guest, 2002: Evaluation of data sets used to force sea ice models in the Arctic Ocean. J. Geophys Res., 107, art. no 3102.
J. A. Curry, et al., 2004: SEA-FLUX. Bull. Amer. Met. Soc., 85 (3), 409–424.
I also note that Roger Pielke Sr has been investigating this issue over land, I don’t have the relevant references handy, hopefully he will spot this and provide some in the comments. The bottom line is that ‘height matters’.
Statistics of the hiatus
Just published in Climatic Change (open access):
Debunking the climate change hiatus
Bala Rajaratnam , Joseph Romano, Michael Tsiang, Noah S. Diffenbaugh
Abstract. The reported “hiatus” in the warming of the global climate system during this century has been the subject of intense scientific and public debate, with implications ranging from scientific understanding of the global climate sensitivity to the rate in which greenhouse gas emissions would need to be curbed in order to meet the United Nations global warming target. A number of scientific hypotheses have been put forward to explain the hiatus, including both physical climate processes and data artifacts. However, despite the intense focus on the hiatus in both the scientific and public arenas, rigorous statistical assessment of the uniqueness of the recent temperature time-series within the context of the long-term record has been limited. We apply a rigorous, comprehensive statistical analysis of global temperature data that goes beyond simple linear models to account for temporal dependence and selection effects. We use this framework to test whether the recent period has demonstrated i) a hiatus in the trend in global temperatures, ii) a temperature trend that is statistically distinct from trends prior to the hiatus period, iii) a “stalling” of the global mean temperature, and iv) a change in the distribution of the year-to-year temperature increases. We find compelling evidence that recent claims of a “hiatus” in global warming lack sound scientific basis. Our analysis reveals that there is no hiatus in the increase in the global mean temperature, no statistically significant difference in trends, no stalling of the global mean temperature, and no change in year-to-year temperature increases.
JC comment: There may be some useful statistical tests, etc. here (I didn’t wade through the supplementary material), but the way they frame the analysis doesn’t make much sense to me.
Lew and Oreskes
And now for the nonsense part.
Stephan Lewandowsky, James Risbey, Naomi Oreskes
Abstract. There has been much recent published research about a putative “pause” or “hiatus” in global warming. We show that there are frequent fluctuations in the rate of warming around a longer-term warming trend, and that there is no evidence that identifies the recent period as unique or particularly unusual. In confirmation, we show that the notion of a “pause” in warming is considered to be misleading in a blind expert test. Nonetheless, the most recent fluctuation about the longer-term trend has been regarded by many as an explanatory challenge that climate science must resolve. This departs from long-standing practice, insofar as scientists have long recognized that the climate fluctuates, that linear increases in CO2 do not produce linear trends in global warming, and that 15-year (or shorter) periods are not diagnostic of long-term trends. We suggest that the repetition of the “warming has paused” message by contrarians was adopted by the scientific community in its problem-solving and answer-seeking role and has led to undue focus on, and mislabeling of, a recent fluctuation. We present an alternative framing that could have avoided inadvertently reinforcing a misleading claim. Capsule: Contrarian discourse about a “pause” in global warming has found traction in climate science even though there is little evidence for anything but a fluctuation in the warming rate similar to earlier deviations from a longer-term trend.
JC comment: I don’t even know where to start on this. I am just astonished that this was published by the Bulletin of the American Meteorological Society. The AMS is the one professional society that I am still somewhat chauvinistic about. Their journals and editorial practices have been superb, and I can’t remember the last AMS paper that I thought “what the heck were the editors thinking?” (note I think this about once a month for Nature Climate Change).
BishopHill has a good post on the paper, with some good comments including one from Nic Lewis. This post is long enough and I am running out of time, so I won’t comment further on this.
Hiatus revisionism is trying to pretend the hiatus doesn’t exist. Sorry, not going to work. There are three key issues:
Observations. We need to look at all of these data sets to understand what is going on with the hiatus:
- Conventional surface temperature analyses. But in light of the new Cowtan et al. paper, I think the land and ocean surface temperature data sets should be analyzed separately and (compared separately with climate models.)
- Satellite atmospheric temperatures (UAH, RSS, etc)
- Satellite ocean temperatures (OISST)
- Upper ocean heat content (ARGO, etc)
- Reanlyses from numerical weather prediction data assimilation systems (ECMWF, CFSR, etc)
- Arctic temperatures from arctic researchers that includes research datasets.
While it is conceivable that 2015 will be unambiguously the warmest year in all of these data sets, we need more realistic uncertainty estimates and reconciliations or at least understanding of differences among the different data sets.
Model-obs comparison. The problem raised by Cowtan et al. is a valid one, but I’m not buying their solution. More in depth comparisons of individual models using different external forcings, and evaluations against all of the above data sets is needed to sort this out.
Statistics. I am all in favor of getting statisticians involved in these evaluations, but they need to work as part of team with climate scientists so that the problem is framed appropriately. And there is plenty of grounds for disagreement on how to do this, but we need to open up this potential can of worms.
And finally, I am most definitely not in favor of advocate historians and psychologists making proclamations about topics in climate science where they seem to have little understanding.