by Judith Curry
Question of the week:
Has the rate of warming continued unabated, or has there been a pause in the warming?
Actually, four different questions seem to be floating around in terms of the BEST media coverage:
- Has the earth been warming? Addressing this question in a sensible way requires that a specific period be specified, presumably in the context of the increase in atmospheric CO2 concentrations.
- Is global warming over? Addressing this question requires a prediction of future temperatures, and we can’t really answer that with the data.
- Has global warming stopped? Addressing this questions requires clarifying whether it is only the actual global temperature under discussion, or whether it is the attribution to humans that has stopped (i.e. beyond the expected range of natural variability)?
- Has there been a pause in the global warming? Addressing this question requires a clarification of of the specific period of interest, and whether the “pause” indicates zero temperature change, or a rate of warming that is less than the expected temperature change.
What does the BEST land temperature data have to say about global warming? Not much, since the BEST data only covers the land (~30% of the global area). The BEST land data should not be used to infer anything quantitative about GLOBAL warming.
IMO, the significance of the BEST data in terms of the temperature record of the past 50 years or so is that it puts to rest the concern that Phil Jones and Jim Hansen have “cooked” the land surface temperature data. This has not been a serious concern among the people paying close attention to this issue and who actually read the journal publications and look at the actual data; but it is a concern in certain circles, and in the U.S. this concern has been raised by at least one Republican presidential candidate. The relatively small discrepancies between the BEST and the GISS and CRU data sets are of some interest; the apparent discrepancy with GISS has been resolved. Note: the CRU data set shows less warming than BEST over the past 15 years.
Back to the 4 questions, which require consideration of the GLOBAL surface temperature data. In terms of simple yes or no answers, there may be different answers to each of these 4 questions. Addressing each of these questions in a sensible way requires analyzing different periods of the data record. But most importantly, they require analysis of the global data, particularly the ocean data.
Further, addressing these questions requires an unambiguous definition of ‘warming’, ‘stopped’, and ‘paused’. ’Warming’ means a rate of change of temperature that is greater than zero. Here I define “stopped” to mean a rate of change of temperature that is less than or equal to zero. Here I define “pause” to mean a rate of increase of temperature that is less than 0.17 – 0.2 C/decade. Why 0.17 -0.2 C/decade as a threshold? From the IPCC AR4 re global temperature trends:
- Observed temperature trends (C/decade) for the period 1979-2004: CRU 0.163; NCDC 0.174; GISS 0.170 (Chapter 3)
- Observed temperature trend (C/decade) 1981-2005: 0.177 (FAQ 3.1)
- “For the next two decades, a warming of about 0.2°C per decade is projected for a range of SRES emission scenarios.” (Summary for Policy Makers)
So what is the evidence for suggesting a ‘pause’ or even a ‘stop’ during the past decade or so?
- HadCRUT3 global temperatures (1998-2010) [link]
- 0-700 m global ocean heat content (2002-present) [link]
- ERSSTv2 global sea surface temperatures (1998-2010) [link]
Note: for reference, the global ocean SST temperature trend (C per decade) for 1979-2005 was reported by the IPCC to be 0.134.
A key issue in identifying and interpreting the pause is the start date chosen to evaluate a pause. If one is seeking to identify an anthropogenic signal, one should choose years at each end point that are neutral in terms of ENSO and also the 9.1 year AMO signal discussed by Muller et al. For a short temperature record (i.e. of relevance to assessing whether there has been a pause over the past decade), this isn’t feasible. In any event, identifying an AGW signal on this short timescale isn’t useful. What is of interest on this timescale is whether natural variability (forced and unforced) can dominate the AGW signal on decadal timescales and produce a ‘pause’ or a ‘stop’. This is the issue addressed by Santer et al., searching for the AGW signal amidst the natural variability noise. Santer et al. argue that “Our results show that temperature records of at least 17 years in length are required for identifying human effects on global-mean tropospheric temperature.”
So in this context, starting the analysis in 1998 is not unreasonable. I have also become intrigued by the varying magnitudes of the 1998 anomaly spike among the different datasets. Among the GISS, HADCRUT, BEST, NCDC, ERSST, and NODC datasets, it is only HADCRUT that has a large 1998 spike. The magnitude of the spike is larger over land than ocean, and also largest in the Southern Hemisphere. Understanding the large discrepancy in the 1998 anomaly among the different datasets (also land/ocean and regional) seems to me like an important thing to pursue.
Another rationale for a starting point is the analysis of Tsonis, who argues for a climate shift ca. 2001/2002, based upon synchronized chaos in the context of the major ocean oscillations. This latest climate shift is characterized by increased frequency of La Nina events and a break in the global mean temperature trend.
This concept of a recent pause in the warming seems to be fairly widely accepted by many mainstream consensus scientists (e.g. the recent Greenwire article),with explanations ranging from aerosols, to solar, to oceans. The duration and magnitude of a pause that is significant in the context of the AGW debate is debatable, but I have made some suggestions. Note that the short time scales considered here preclude determination of a statistically significant trend at the 95% confidence level, although lack of statistical signficance does not negate the existence of a pause as defined here.
This issue of ‘pause’ has generated a plethora of blogoshperic analyses of the recent record in the Berkeley Earth and other data sets (much traffic for woodfortrees). Statistical analyses of various data sets over various periods aren’t all that interesting in the absence of an hypothesis that is related to a physical mechanism. I hope that this post stimulates more meaningful analyses of the recent data record.