In an opinion article in the Sunday Salt Lake Tribune (Stewart Cautious on Climate Change), Rep Chris Stewart, newly appointed chairman of the House Sub-Committee on the Environment, argues that “the science regarding climate change is anything but settled” and that “as a leader on these important issues, I will look to ensure that we have conducted a thorough scientific review, then use that information to advocate for reasonable policy decisions.” As an atmospheric scientist and voter in Rep. Stewart’s district, I agree with both of these statements, but ultimately conclude that the time for action is now (or arguably a few decades ago).
Rep. Stewart is correct that the science regarding climate isn’t settled. As is the case in any science, there are always limits to knowledge, and scientists are constantly testing hypotheses, collecting new data, and working hard to obtain a greater understanding. Knowledge is never absolute, and uncertainty is considered in everything that we do. This is reflected in key synthesis reports that summarize current understanding of climate change, which I have summarized below. When not present in the original document, I have italicized phrases that indicate scientific confidence or probabilities.
• “Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.” (IPCC 2007)My interpretation of these statements and the hundreds of scientific papers that I have read regarding climate change is that dramatic changes are occurring to the climate system, that a preponderance of evidence shows that there is a strong human-driven component to this change, and that we are very likely to see an even larger increase in temperature during the 21st century if we continue to conduct "business as usual." Rep. Stewart agrees that the climate is changing, but argues that this has always been the case and that there is no ideal temperature that the earth is trying to achieve. This perspective, however, ignores the fact that our society is built around the climate of the 20th century, and therefore, at minimum we should be considering climate change in long-term planning and adaptation efforts.
• “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.” (IPCC 2007)
• “The observed widespread warming of the atmosphere and ocean, together with ice mass loss, support the conclusion that it is extremely unlikely that global climate change of the past 50 years can be explained without external forcing, and very likely that it is not due to natural causes alone.” (IPCC 2007).
• “It is very likely that there has been an overall decrease in the number of cold days and nights, and an overall increase in the number of warm days and nights, at the global scale, that is for most land areas with sufficient data.” (IPCC 2012)
• “There have been statistically significant trends in the number of heavy precipitation events in some regions. It is likely that more of these regions have experienced increases than decreases, although there are strong regional and subregional variations in these trends.” (IPCC 2012)
• “There is low confidence in any observed long-term (i.e., 40 years or more) increases in tropical cyclone activity (i.e., intensity, frequency, duration), after accounting for past changes in observing capabilities. It is likely that there has been a poleward shift in the main Northern and Southern Hemisphere extratropical storm tracks. There is low confidence in observed trends in small spatial-scale phenomena such as tornadoes and hail because of data inhomogeneities and inadequacies in monitoring systems.” (IPCC 2012)
• Since 1950, “there is medium confidence that some regions of the world have experienced more intense and longer droughts, in particular in southern Europe and West Africa, but in some regions droughts have become less frequent, less intense, or shorter, for example, in central North America and northwestern Australia.” (IPCC 2012)
• “Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.” (IPCC 2007)
The statements above also reflect the fact that linkages between climate change and so-called “extreme weather” vary depending on the phenomenon. Based on the evidence available to date, there is a well-established link between climate change and the increased frequency of warm days and nights, as well as heat waves, as well as decreased frequency of cold days and nights. There is mounting evidence of a linkage to the frequency and intensity of large precipitation events, but conclusions in this area are limited by the infrequent nature of these events combined with the limited quantity and quality of precipitation data, especially in areas outside the developed world. Linkages with trends in tornadoes and tropical cyclones are less clear and, at least for tornadoes, it is unclear if we should even expect a trend.
Thus, I agree with Rep. Stewart that there have been outlandish connections made between climate change and individual weather events by some groups. Extreme weather events occur when the atmosphere is in outlier mode and nearly always have a “natural” component. Separating natural from anthropogenic effects during individual events is incredibly difficult and likely to remain a subject of debate far into the future. I simply don’t see this as a good litmus test for whether or not humans are causing climate change, or as a good indicator of how severe climate change might be in the future.
Finally, Rep. Stewart raises concerns about the fidelity of current climate models. Statistician George Box once said that “all models are wrong, but some are useful,” and this is my motto in both weather and climate prediction. My concern about future climate change is based not only on my assessment of climate model fidelity (which is not as dour as Rep. Stewart’s) and their projections for the 21st century, but also our theoretical understanding of the climate system, past climate change, and trends in recent decades not only in atmospheric temperatures, but also snow, ice, ocean temperatures, and other indicators. This assessment leads me to a conclusion similar to that made by M in the Bond film Skyfall:
“Well, I suppose I see a different world than you do, and the truth is that what I see frightens me.”It frightens me not because I'm certain that we will see disruptive climate change during the 21st century, but because it is within what I consider to be reasonable assessment of what is possible. Of course, that assessment does include the possibility of a more gradual climate change that might be less disruptive. At issue is whether or not we should bet on that less disruptive outcome (which I see as a low probability possibility under business as usual) and continue with the status quo, or if we could instead enact policy measures to stimulate innovation and the migration to a low-carbon, high-efficiency energy future without negative economic consequences. I'd like to think the latter is possible, but it lies outside my area of expertise.