Public opinion surveying consistently indicates* that large portions of the U.S. population understand that global warming is happening and occurring at a troubling pace. By a ratio of six-to-one (72% to 12%), the public accepts global warming as a reality.

Six-to-one odds like that would be widely accepted as good news in any sports contest or election. So much so that some wonder what factors are driving that dismissive 12%, or the remaining 16% who declare that they just don’t know.

One contributing factor is the difficulty of understanding our very noisy planet based on an individual’s personal experiences. What people see is the weather – daily temperature, rain and snowfall, and an occasional storm – all of which vary naturally day to day, season to season, year to year, and place to place.

Climate change, in contrast, involves the progressive shift in these patterns over decades, and few people can clearly remember what July was like 30 or 40 years ago. Occasionally an extreme weather event may shake attitudes about climate change, but the first step in appreciating the climate threat is acceptance of evidence based on over a century of measurements.

So what is this evidence? Several groups prepare estimates of the path of global temperature since the beginning of the industrial age. Two of these are in the U.S.: NASA’s Goddard Institute for Space Studies and the National Oceanic and Atmospheric Administration (NOAA). Others include the UK Met Office’s Hadley Centre, the Japan Meteorological Agency, and several non-government teams.

Each of these science-based groups combines information from numerous datasets, including air temperature (e.g., from long records of meteorological agencies), ocean surface temperature collected by ships and specialized buoys, and (since the 1970s) land and ocean temperature measures derived from data collected by satellites.

Somewhat different methods are applied in these studies as findings from different sources are combined into a global picture. But despite these differences their estimates don’t vary much. Results from several of the groups for the increased global average temperature over the past 140 years are shown in Figure 1. Over this period the planet has warmed by a bit more than 1 degree Celsius (slightly more than 2 degrees Fahrenheit). But even more troubling than the 140-year change is the rapid increase over just the past 50 years.

Every place on the globe does not warm at the same pace, of course. There is substantial variability in temperature by region over time, with the poles, particularly the North Pole, seeing the most rapid warming. That pattern is strikingly illustrated by dynamic plots of the sequence of change over the period.

But even with substantial year-to-year temperature variation at a global scale, as evident in Figure 1, five of the hottest years on record have occurred since 2015. Moreover, Earth is on a path to continued warming. This last observation raises a question of what is causing this change, and the upcoming essay in this series, to be posted September 25, will show how peer-reviewed science attributes most of the observed warming to human activities that emit heat-trapping gases.

It is not only the change in average warming that is a concern, of course, but also what is happening to temperature highs and lows. In any period, say a month, the daily temperature ranges above or below the monthly average, as illustrated in Figure 2. Local weather forecasters’ comparisons of the day’s high or low temperature with the average for the particular time of year are staples of local TV weather reports. As the average temperature rises, the number of unusually hot days should be expected to go up, with fewer very cold days. Indeed, that is what is happening,

Consider, for instance, the U.S. experience. At each of thousands of weather stations located around the country, special note is taken when a new high or low daily temperature is recorded, and the same for a new high or low monthly average. In the past year, the number of new record monthly highs exceed new record lows by two to one. The October 25 essay in this series will explore implications of a continued breaking of high-temperature records that comes with the warming in Figure 1, as illustrated in Figure 2.

The 2020 disastrous late-summer heat wave in the western U.S. no doubt will add to the growing collection of broken temperature records. However, individual heat events like this, though more likely with rising average temperatures, are not a good basis for swaying those dismissive of data that Earth is warming, or convincing those who are still not sure. All too soon, the planet’s noisy weather system will provide a contrasting event far from the average – perhaps a Washington D.C. snowfall – to be used as evidence against warming by those determined to oppose climate action.

The key to understanding the climate change threat is in the long temperature record, requiring only confidence in thermometers – the temperature records collected by thousands and thousands of people in countries worldwide, and pulled into a global picture by several independent analysis groups.

*The lead researcher for this report directs the Yale program that publishes this site. 

Henry Jacoby is the William F. Pounds Professor of Management, Emeritus, in the MIT Sloan School of Management and former co-director of the MIT Joint Program on the Science and Policy of Global Change, which is focused on the integration of the natural and social sciences and policy analysis in application to the threat of global climate.

Gary Yohe is the Huffington Foundation Professor of Economics and Environmental Studies, Emeritus, at Wesleyan University in Connecticut. He served as convening lead author for multiple chapters and the Synthesis Report for the IPCC from 1990 through 2014 and was vice-chair of the Third U.S. National Climate Assessment.

Richard Richels directed climate change research at the Electric Power Research Institute (EPRI). He served as lead author for multiple chapters of the IPCC in the areas of mitigation, impacts and adaptation from 1992 through 2014. He also served on the National Assessment Synthesis Team for the first U.S. National Climate Assessment.

Additional posts in this series:

– Inaction on the climate threat is NOT an option
– Rejoining the global fight against climate change: In the U.S.’s national interest
– Vigorous action needed, and soon, on climate change
– Multiple extreme climate events can combine to produce catastrophic damages
– Extreme events ‘presage worse to come’ in a warming climate
– The evidence is compelling on human activity as the principal cause of global warming
– Evidence shows troubling warming of the planet
– Key messages about climate change: an introduction to a series
– Five science questions that ought to be asked at the debates