I write to clarify issues related to changes in the heat content of the Earth’s atmosphere and its relationship to climate change. It has recently been claimed by some that increases in atmospheric carbon dioxide (CO2) during the last 70 years has had no measurable impact on the temperature of the Earth.
Climate change deniers argue that there has been a “pause” in the rate of global warming during the last decade. In fact, some claim that the Earth has actually cooled by a mean of 0.7 degrees Celsius (°C) in recent years. However, numerous studies (e.g., Risbey et al. 2018. A fluctuation in surface temperature in historical context: reassessment and retrospective on the evidence. Environ. Res. Lett., 13: 123008) have disproved this claim. The evidence of a warming trend over the past century is unequivocal. Here are the realities:
Between 1880 (the year that accurate record keeping began) and 1980, the Earth’s atmosphere warmed by an average of 0.07°C every 10 years. However, since 1981, annual mean global temperature has increased by 0.18 °C per decade – more than double that of the previous 100 years in just 40 years! In this context, it is important to note that these decadal trends are occurring against a backdrop of natural temperature variability due to seasonal changes in solar radiation and changes in ocean circulation (e.g., fluctuations between La Niña and El Niño events). Thus, for example, global temperatures in 2021 are running cooler when compared to recent years due to a moderate La Niña event in the tropical Pacific Ocean,
Although the mean annual temperature of the atmospheric has varied from year to year, the amount of CO2 in the atmosphere has increased steadily over the last 60 years with little seasonal or inter-annual variability. There is more CO2 in the atmosphere now than at any time in the last 3 million years. According to the National Oceanic and Atmospheric Administration, the amount of atmospheric CO2 in May 2020 hit an average of about 417 parts per million (ppm). This is the highest monthly average value ever recorded, and is up from 415 ppm in May of 2019. The last time global CO2 levels in the atmosphere exceeded 400 ppm was around four million years ago! The result? Nine of the 10 warmest years since 1880 have occurred since 2005, and the 5 warmest years on record have all occurred since 2015.
Finally, one must distinguish between global warming and climate change. Global warming and climate change are not synonyms. “Climate change” includes global warming, but also refers to a broader range of changes that are occurring globally, e.g. increases in the strength of extreme weather events (tropical storms, droughts, floods); changes in global patterns of precipitation, winds and ocean currents; and melting ice caps and sea level rise. These are all consequences of global warming that will profoundly impact our health and wellbeing in the long term.
Tom Malone, Ph.D. is Professor Emeritus at the University of Maryland Center for Environmental Science