"Apples and Oranges": On comparing simulated historic near‐surface temperature changes with observations

Abstract

Simulated historic near-surface air temperature variations are often compared with observations of land air temperatures blended with sea surface temperatures. This study investigates claims that this is not a "true like-with-like" comparison, which may cause small biases in simulated twentieth century temperature changes, with implications for different climate attribution and projection studies. A more appropriate analysis, it is claimed, should use simulated sea surface temperatures blended with land air temperatures; an apparent discrepancy with observed trends is then reduced. As the temperature of the uppermost level in a model's ocean is used to represent simulated sea surface temperatures, that models have inconsistent ways of representing land, and that simulations have differing sea ice coverages, the claim of an idealised analysis approach is challenged. An examination of Coupled Model Intercomparison Project simulations, compared with an observational dataset of near-surface temperatures, suggests there is a bias in simulated historic trends when upper-ocean temperatures are used instead of marine air temperatures, but this bias is small compared to other model and observational uncertainties and the impact of analysis choices. The results indicate that it is generally appropriate to use global near-surface air temperature diagnostics to compare simulated historic climate change with observed temperature changes. Alternative model diagnostics are not necessarily superior to those used in standard approaches, and the emphasis of model and observational discrepancies may be based on overconfident reasoning. K E Y W O R D S climate change, climate models, observed temperatures, CMIP5 1 INTRODUCTION The assessing of past and future changes in climate has a long history (Le Treut et al., 2007; Bindoff et al., 2013; Collins et al., 2013), which has been dominated by the use of surface temperatures, as measurement records are particularly long, have reasonable global coverages, and arguably have the most important impact on our lives. Multiple datasets indicate that global mean near-surface temperatures have increased by about 1 K since the end This article is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.

Global Climate

“…The NMAT-SST discontinuity may appear as a step-like change in the early 1990s (Kennedy et al 2019), although this may be a manifestation of a long-term divergence between SST and NMAT coupled with the use of a common 1961-90 base period for the calculation of the anomalies. In the evaluation of the long-term trends in NMAT and SST, differences in spatial coverage may have a large influence on the results (Jones 2020). In general, SST is more spatially complete than NMAT.…”

“…Understanding this feature is particularly important because global mean surface temperature (GMST) data products (Lenssen et al 2019;Morice et al 2021;Vose et al 2012) combine anomalies of near-surface temperature over land with anomalies of SST rather than MAT. Resolving this question would also inform the debate about the suitability of comparing these merged GMST datasets against global climate model simulations of air temperature (Cowtan et al 2015;Jones 2020), especially since simulated values using MAT for the marine component of global air temperature have been shown to warm at a slightly faster rate than a comparable dataset that used SST as the marine component (Richardson et al 2018) These analyses of NMAT and MAT illustrate the importance of exploring many different variables, using as many different methods as possible, and that some questions are yet unanswered. Despite the various issues discussed and differences in long-term trend, NMAT and MAT show similar year-to-year variability to spatially-matched SST in terms of the global average time series (Figs.…”

The Berkeley Earth Land/Ocean Temperature Record

“…1750 for land (Rohde et al, 2013a), and it is the part of the Earth system most relevant for impacts on human civilization. Sea surface temperatures are used in lieu of marine air temperatures due to scarcity and inhomogeneity of marine air temperature data (Kent et al, 2013), though it is only an imperfect proxy and may be subject to slightly slower warming rates than marine air temperatures in recent decades (Cowtan et al, 2015;Richardson et al, 2016;Jones, 2020).…”

Quantifying Daytime Heating Biases in Marine Air Temperature Observations from Ships

“…The ultimate goal is to use these adjusted data to create a GSAT record based on air temperature over land, ice, and ocean. This will facilitate comparison of the observed surface temperature record with the output of climate models (Jones 2020), which most straightforwardly provide estimates of GSAT rather than GMST.…”