SFU Faculty of Science: Global Warming Seminar Series "The Instrumental Temperature Record and What it Tells us About Climate Change"

Abstract

The modern instrumental surface temperature record extends back in time over about one and a half centuries. Since 1850, the number of points at which temperature is observed has increased enormously although even today, there remain many places on Earth where temperature is not routinely observed and reported. Such data gaps do not pose a serious impediment for the reliable estimation of changes in global mean temperature, as has been demonstrated by both empirical and theoretical research. A greater potential concern is that temperature observations are often affected by non-climatic influences including changes in instrumentation, instrument exposure, and instrument location. A great deal of work has been done to remove or avoid those influences wherever possible. Urban heat islands affect some temperature measurements taken over land, but do not substantial affect trends in the global mean record. Estimates of global mean temperature from surface temperature compilations produced by several different research groups are similar, demonstrating robustness to the specific choices that are made in their development. The resulting global record has been studied extensively, and is considered to be reliable. The record shows an overall warming combined with low- and high-frequency variability. That there is warming is indisputable and is supported by additional lines of evidence (such as cryosphere changes, ocean heat content increases, and sea level rise). Much variability results from internal processes in the climate system and natural external forcing, such as volcanic activity and solar output changes. However, these factors alone do not explain the observed temperature changes well; radiative forcing caused by increases in greenhouse gas concentrations and changes in aerosol loadings provides a more plausible explanation. Statistical comparisons between observed temperature changes and those simulated by climate models that take various external forcing agents into account, as well as analyses of other possible causes, led to the IPCC 2007 conclusion that "most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations".