Some skeptics have seized on a recent article in The Economist noting an apparent “hiatus” in global warming to argue that climate change is a fiction and efforts to address it are misguided. Those interpretations, which were voiced by some representatives at a recent House hearing on climate science, misrepresent both the article and the science it examines.
So what are the facts?
Zooming in on one day, one year or even one decade misses the larger picture emerging over the last century – that the concentration of greenhouse gases in the atmosphere is increasing and that average global temperatures have been rising. As a result, sea level is higher and climate scientists tell us to expect increasingly more frequent heat waves and heavy rainfall events. These changes can contribute to more frequent and severe flooding, more intense droughts, and more widespread wildfires.
The Economist article notes that over the past 15 years, air temperatures at the Earth’s surface have been flat. Although this is true, temperatures are still significantly higher than they were 30, 50, or 100 years ago – almost 1 degree Celsius above average temperatures in the first decade of the last century. And all the years in the 21st century (2001-2012) rank in the top 14 warmest of the 133-year instrumental record (See figure).
Throughout this warming trend, there have been ups and downs and, yes, flat periods, reflecting a host of factors, including volcanic eruptions and the movement of heat between the atmosphere and the ocean. As noted by the Economist, these factors may at times mask or undercut the effect of global warming; at other times, they may enhance it.
In particular, several recent studies (here, here, and here) suggest that the lack of recent warming is consistent with an increase in heat uptake by the oceans. In other words, the oceans are absorbing more of the excess heat in our atmosphere. Although the precise roles of the upper and deep ocean are still active research questions, these findings demonstrate that a “warming hiatus” in air temperatures does not suggest that global warming has stopped.
One issue that pervades the article: a mistranslation of the concept of “climate sensitivity,” which represents the amount of warming expected to occur if the concentration of carbon dioxide in the atmosphere doubles. It’s an estimate for long-term “equilibration” of the climate system, which is reached after many centuries. The article incorrectly interprets “sensitivity” as a prediction for global temperature on an annual or decadal timescale.
The bottom line is that greenhouse gases continue to accumulate in the atmosphere, raising global mean temperature and altering many weather patterns. Communities and businesses are already experiencing significant and costly impacts, which are likely to continue, absent significant steps to mitigate these emissions or adapt to the new and changing climate conditions.
Figure 1: Global temperature from 1950-2102 show significant warming occurring in the past 50 years. El Niño years, which favor warmer global temperatures, and La Niña years, which have historically been associated with relatively cooler global temperatures, are shown in color. Source: NOAA National Climate Data Center.