In 2005, I changed my mind about climate change: I concluded that the balance of the scientific evidence showed that man-made global warming could likely pose a significant problem for humanity by the end of this century. My new assessment did not please a number of my friends, some of whom made their disappointment clear.
At the 2007 annual gala dinner of the Competitive Enterprise Institute, a D.C.-based free-market think tank, the master of ceremonies was former National Review editor John O'Sullivan. To entertain the crowd, O'Sullivan put together a counterfeit tale in which I ostensibly had given a lecture on environmental trends pointing out that most were positive. After my talk, O'Sullivan told the audience, a young woman supposedly approached me to express her displeasure with regard to my change of mind on climate change.
Continuing his fable, O'Sullivan recounted to the hundreds of diners that I had tried to explain why my views had shifted. Eventually realizing that the young woman was having none of it, I then purportedly asked her if it wasn't enough that we two actually agreed on most environmental policy issues. The young woman paused for a moment, said O'Sullivan, and then retorted, "I suppose that Pontius Pilate made some good decisions, too." Being compared, even in jest, to the Roman governor who consented to the crucifixion of Jesus is, to say the least, somewhat disconcerting.
Welcome to the most politicized science of our time.
So what evidence would convince you that man-made climate change is possibly real? Keep in mind that despite what progressive dimwits like Naomi Klein might assert, the scientific evidence does not mandate any particular program.
What about higher temperatures? Obviously, in order for there to be any man-made global warming, temperatures must be going up. Are they? Yes.
Concentrations of greenhouse gases in the atmosphere have increased from 280 parts per million in the late 18th century to around 400 ppm today. And the trend in average global surface temperatures has been increasing since the late 19th century. As I've reported before, all of the global temperature datasets, both the instrumental and satellite, find that the atmosphere has warmed since the 1950s.
By how much? Summed over the past 35 years—that is, since the advent of satellite monitoring—temperatures have increased by at most 0.56 C° (1 F°) and at least by 0.455 C° (0.8 F°). In general, the instrumental records suggest that surface temperatures have warmed on average by about +0.9 C° (1.6 F°) since the 1950s.
Let's look at the near-term trends. The average rate of increase since 1979 varies among the temperature datasets from a high of +0.16 C° to a low of +0.13 C° per decade. The rate of surface temperature increase dramatically slowed after 1998 to rate of around +0.05 C° per decade. Of course, correlation does not imply causation, but how sure can you be that the rise in the atmospheric concentration of greenhouse gases just happens to coincide with an entirely natural increase in average temperatures? Conversely, how sure can you be that a natural decline in average temperatures is not temporarily countering a trend toward to higher temperatures caused by accumulating greenhouse gases? Explanations based on natural variability work both ways. I will address the recent "hiatus" in temperature trends below.
What about converging daytime and nighttime temperatures?
Climatologists predicted that man-made warming would produce a decrease in the differences between low nighttime temperatures and high daytime temperatures. And indeed, a decrease between day and night temperatures has been occurring in the United States, China, Spain, and other regions. This phenomenon is global, although more recently daytime and nighttime temperatures have been increasing at about the same rate. Along with the observed increases in average temperature, heat waves have become more common since the 1950s.
What about earlier spring and later fall seasons?
Many studies find that the onset of spring is occurring earlier than it did decades ago. A 2015 study reports that the advent of spring in the Northern Hemisphere occurs about 4 days earlier than in 1980. A 2006 European study found that spring is arriving about 3 days earlier, and a 2014 study reported that the growing season in the Northern Hemisphere is expanding.
Part of the reason that spring is advancing is that the extent of snow cover in March and April in the Northern Hemisphere has been falling. As a 2011 study in the journal Cryosphere reports, "The rate of decrease in March and April Northern Hemisphere (NH) Snow Cover Extent (SCE) over the 1970–2010 period is ~0.8 million km2 per decade corresponding to a 7% and 11% decrease in NH March and April SCE respectively from pre-1970 values." The decline in snow cover is broadly in line with climate model predictions.
What about disappearing glaciers and Arctic sea ice?
The Arctic-wide melt season has lengthened at a rate of 5?days per decade from 1979 to 2013, according to a 2014 study in Geophysical Research Letters. A 2014 review article looks at what satellite data are telling us about recent climate trends in the Arctic. Temperatures are rising at 0.6°C per decade, about 4 times the global average. Sea ice extent has been falling at 3.8 percent per decade, and spring snow cover is dropping by 2.1 percent per decade. The Greenland ice sheet has been losing mass at a rate of 34 gigatons per year, though that has increased sevenfold since 2002 to an estimated 215 gigatons per year.
The growing extent of sea ice in the Antarctic over the past decades is a climate change conundrum. On the face of it, more sea ice would indicate cooling rather than warming. Researchers are still trying to figure out what is going on. One idea is that warmer waters are melting the bases of freshwater Antarctic ice shelves. The fresh water then cools the sea surface thus promoting the freezing of more sea ice. When climate researchers don't understand what is going on they often attribute the empirical trends to "internal variability."
What about stronger rainstorms?
As temperatures increase by 1 degree Celsius, global average water vapor in the atmosphere is estimated to increase by around 7 percent. It is difficult to determine the average global humidity. But a 2005 study parsing satellite data finds that the atmosphere did moisten, as predicted, between 1982 and 2004. A 2014 study confirmed the finding and suggests that the increase is mostly the result of man-made warming.
Increased atmospheric humidity suggests that precipitation should also increase. The data show that this is happening. A 2013 study that analyzed data from nearly 9,000 weather stations from around the globe found increases in annual maximum daily precipitation at nearly two-thirds of the stations since 1900. (Climate change does not appear to be exacerbating hurricanes, tornadoes, or droughts.)
What about warming oceans?
Does the recent 17-year hiatus in rising global temperatures cut strongly against the notion of man-made global warming? The pause certainly was not predicted by the computer climate models. As the researchers at the private consultancy Remote Sensing Systems have noted, "The troposphere has not [their emphasis] warmed as fast as almost all climate models predict." University of Alabama in Huntsville climatologist John Christy compared 102 climate model predictions with actual temperature data and found that "their response to CO2 on average is 2 to 5 times greater than reality." Pretty damning.
Other researchers have reluctantly come to acknowledge that there has been a slowdown in surface temperatures. But while surface temperatures may be on pause, they are convinced that "global heating" is not. Lots of researchers have been reporting that for the past couple of decades, 90 percent of the extra heat from greenhouse warming has been sequestered in the oceans. In February, Nature Climate Change asserted that planetary warming continues "unabated," with most of the excess heat being absorbed by the top 2,000 meters of the oceans. Just how and where the heat gets buried in the oceans remains controversial.
Last year an intriguing study in Science suggested that natural variability in the North Atlantic can keep transporting heat downward into the deep ocean for periods lasting 20 to 35 years. Those researchers propose that "the latter part of the 20th century saw rapid global warming as more heat stayed near the surface. In the 21st century, surface warming slowed as more heat moved into deeper oceans."
How about some falsifiable predictions?
Another February 2015 article in Nature Climate Change makes the bold prediction that the current hiatus will likely last only until the end of this decade. Around 2020, the authors suggest, the oceans will start to release the stored heat and surface temperatures will begin to rise rapidly. An even more alarming (alarmist?) article in the April 2015 Nature Climate Change asserts that the rate global average temperature increases will rise to 0.25°C per decade by 2020, "an average greater than the peak rates of change during the previous one to two millennia."
The future course of man-made warming depends on climate sensitivity, conventionally measured as how high average temperature would eventually increase if atmospheric carbon dioxide were doubled. In recent years, there has have a lot of back and forth between researchers trying to refine their estimates of climate sensitivity. At the low end, some researchers think that temperatures would increase a comparatively trivial 1.5 degrees Celsius; on the high end, some worry it could go as high as high 6 degrees Celsius. The uncertainty over this variable is largely why I think that future warming could become a signficant problem. In a 2014 article in Geophysical Research Letters, a group of researchers calculated that it would take another 20 years of temperature observations for us to be confident that climate sensitivity is on the low end and more than 50 years of data to confirm the high end of the projections. How lucky do you feel?
In his magisterial 1960 essay "Why I Am Not A Conservative," economist Friedrich Hayek observed:
Personally, I find that the most objectionable feature of the conservative attitude is its propensity to reject well-substantiated new knowledge because it dislikes some of the consequences which seem to follow from it—or, to put it bluntly, its obscurantism. I will not deny that scientists as much as others are given to fads and fashions and that we have much reason to be cautious in accepting the conclusions that they draw from their latest theories. But the reasons for our reluctance must be rational and must be kept separate from our regret that the new theories upset our cherished beliefs.
What Evidence Would Persuade You That Man-Made Climate Change Is Real?
It might be that it is just so happens that natural climate variability has boosted global temperatures and the trends discussed above are occurring coincidentally at the same time the concentrations of carbon dioxide are 30 percent above their highest levels in the past 800,000 years. Correlation does not imply causation. The data cited (and uncited) do not prove beyond a reasonable doubt that man-made climate change is real. However, in my best judgment the preponderance of the evidence suggests that the greenhouse gases produced by humanity are warming the climate and that it could be a significant issue later in this century. In the foregoing I have aimed to cite data, not model outputs. I have long been a critic of computer climate models.
To restate: The existence of man-made warming does not mandate any particular policies. So back to the headline question: If generally rising temperatures, decreasing diurnal temperature differences, melting glacial and sea ice, smaller snow extent, stronger rainstorms, and warming oceans are not enough to persuade you that man-made climate is occurring, what evidence would be?