A Climate Change Pause? The Science Behind the Debate

Was there a pause in climate change or is our planet heating up? Photo by Steven Depolo.

Within the scientific community empirical acceptance of anthropogenic climate change is by far the prevailing view, with scientific dissent comprising only a small minority. Outside the research community, however, a recurring contention by climate skeptics and oppositional policy-makers is that the veracity of the underlying scientific research is suspect. The most prominent incident in recent memory involved accusations several years ago of data manipulation related to a spate of hacked email communications from the Climate Research Unit at the U.K.’s University of East Anglia (the so-called “climategate” scandal). Such incidents, while occasionally engendering negative public perception (at least among some segments of the population), have not altered the fundamental scientific confirmation that human activity has led to a global rise in temperatures starting in the early- to mid-twentieth century.

However, recent years have seen a new argument among skeptics: namely, that the rise in global temperature has entered a period of hiatus not foreseen by climate models. Indeed, while most models predicted an acceleration of warming concomitant with the accelerated emissions of greenhouse gases, the global surface temperature appears to have slowed significantly since the late 1990s. This observation eventually led to a direct acknowledgement of the unexpected development, by the U.N. Intergovernmental Panel on Climate Change (IPCC) in its most recent Fifth Assessment Report (2013):

“The observed global mean surface temperature (GMST) has shown a much smaller increasing linear trend over the past 15 years than over the past 30 to 60 years… Depending on the observational data set, the GMST trend over 1998-2012 is estimated to be around one-third to one-half of the trend over 1951-2012… Even with this “hiatus” in GMST trend, the decade of the 2000s has been the warmest in the instrumental record of GMST… Nevertheless, the occurrence of the hiatus in GMST trend during the past 15 years raises the two related questions of what has caused it and whether climate models are able to reproduce it.”

The apparent subsidence of warming has been touted by some politicians as evidence that climate modeling is unreliable and that, by extension, the Earth is not really warming. How has the scientific community responded to these new observations? The possibility that global surface temperatures might have flattened prompted a flurry of reports attempting to reconcile the data to climate models (a good review can be found here). Solar activity can impact global temperatures (e.g., the exceptionally weak activity during the Maunder Minimum of the 17^th century), and some research suggests that the minimum of the last solar cycle, which was longer and deeper than prior cycles since accurate measurements began in the 1970s, may have had a negative impact on warming trends. A decrease in stratospheric water vapor and increasing stratospheric aerosols may have counteracted warming in the 21^st century, while some studies indicate that the excess surface heat has moved to the ocean waters. Other researchers have taken a wider perspective of the overall trends in the climate, arguing that global temperatures are subject to multidecadal oscillations that are consistent with periods of warming alternating with temporary pauses.

However, an article published in Science this past June seems to offer the most forceful explanation for the hiatus thus far: that it never occurred at all. Adjustments made by NOAA to its analysis of international sea surface temperature measurements (the Extended Reconstructed Sea Surface Temperature data set) suggest that the pause may have been an artifact, stemming from the increasing dominance of buoy measurements over ship-based measurements across the past few decades (buoys give systematically lower temperature readings). Correcting for this and other time-dependent biases, the authors argue, eliminates the apparent warming hiatus. It remains to be seen how these new findings will be related to satellite-based readings of the troposphere (e.g., through the RSS and UAH), which continue to indicate signs of a slowed rate of increase in the temperature anomaly over recent decades.

In reading the articles mentioned above and others, one is struck by the sheer complexity of efforts to study global climate patterns, and the often contradictory or otherwise difficult-to-reconcile findings resulting from the use of disparate observational data sets and modeling methods. To those of us accustomed to studies conducted at the bench, in which comparatively simple experiments can usually be performed with careful monitoring of confounding variables, discerning conclusions from climate research might seem a herculean undertaking. Indeed, even the correction of temperature records presented in the aforementioned Science paper seem minor to the untrained eye, compared to the oscillatory noise across the century. Whether or not global temperatures have stalled or continue to rise, however, it is important to bear in mind the broader outlook—the accumulated evidence is abundant that changes to the climate have already begun to have major impacts across the globe.

It is essential that scientists and policy-makers provide a persuasive narrative to the public regarding the reality of anthropogenic climate change. Given the particularly strident nature of the global warming debate in the U.S. polity, however, such a narrative should not appear to downplay conflicting data when it arises. The public needs to be reminded that scientific endeavors do in fact undergo continual self-evaluation, but at the same time researchers should readily and openly address shortcomings of climate models. Otherwise, the research community risks leaving itself vulnerable to public skepticism.