One of the most puzzling aspects of the global warming controversy is the divergent responses to the simple question: Have global temperatures been rising in recent years?
Sounds simple enough, yet I keep hearing two very different answers. One side says yes, temperatures have continued to rise; the other side says no, temperatures have not risen for about the past decade.
In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) wrote:
Eleven of the last twelve years (1995-2006) rank among the twelve warmest years in the instrumental record of global surface temperature (since 1850)… The temperature increase is widespread over the globe and is greater at higher northern latitudes.
According to a 2009 report from the U.S. National Oceanic and Atmospheric Administration (NOAA):
A comprehensive review of key climate indicators confirms the world is warming and the past decade was the warmest on record.
But on the other side, Joseph Bast and James M. Taylor, writing for the Heartland Institute, assert:
While the global climate warmed slightly during the 1980s and 1990s, it has not warmed at all since 2000, and there is some evidence that a cooling trend has begun… This contradicts the predictions of the IPCC and poses a challenge to the theory that CO2 concentrations play a major role in global temperature trends.
And in January, 16 prominent figures, including scientists, submitted an opinion piece to The Wall Street Journal, claiming:
The lack of warming for more than a decade — indeed, the smaller-than-predicted warming over the 22 years since the U.N.’s Intergovernmental Panel on Climate Change (IPCC) began issuing projections — suggests that computer models have greatly exaggerated how much warming additional CO2 can cause.
However, these assertions seem at odds with data from NASA, even on a short time scale. According to a report from the agency:
January 2000 to December 2009 was the warmest decade on record. Throughout the last three decades, the GISS surface temperature record shows an upward trend of about [0.4 deg F] per decade. Since 1880 — the year that modern scientific instrumentation became available to monitor temperatures precisely — a clear warming trend is present, though there was a leveling off between the 1940s and 1970s.
Another NASA report says that 2010 matched 2005 as the warmest year on record. The agency says its analysis finds that 2010 was about 1.13 deg F warmer than average temperatures from 1951 to 1980 and that the climate “has warmed by approximately 0.36 deg F per decade since the late 1970s.”
The claims that global warming has stalled are based on data such as the graph below from the Met Office Hadley Centre, which shows global average temperatures from 1850 to 2011:
As you can see, the graph shows average temperatures topping out in the early 2000s and even declining a bit. So the assertions from the Heartland Institute and the Wall Street Journal signatories seem to have some basis.
Confusion Over Time Scales
From everything I can tell, the question whether global temperatures are rising is not particularly controversial, especially at longer time scales. Christopher Monckton, a prominent climate-change critic, says, “Of course the Earth has been warming since 1750: I have at no point denied it …” What he charges is that the principal conclusion of the 2007 IPCC report was reached “on the basis of a fraudulent statistical abuse,” i.e., “comparison of the slopes of multiple arbitrarily-chosen trend-lines on the global-temperature dataset falsely to suggest that ‘global warming’ is accelerating and that it is our fault.”
One thing you have to keep in mind is that climate change occurs at multiple scales both in time and geography. Monckton has written: “Strictly speaking, one should study temperature trends in multiples of 60 years, so as to ensure that the warming and cooling phases of the PDO [Pacific Decadal Oscillation] cancel one another out.”
Roy Spencer, a climate scientist, in discussing the question whether global temperatures are rising right now, writes:
There is no way to know, because natural year-to-year variability in global temperature is so large, with warming and cooling occurring all the time. What we can say is that surface and lower atmospheric temperature have risen in the last 30 to 50 years, with most of that warming in the Northern Hemisphere… But there is no way to know if temperatures are continuing to rise now — we only see warming (or cooling) in the rearview mirror, when we look back in time.
Spencer is aware that some researchers say the climate is warming and some say it is cooling. About this apparent contradiction, he brings up the issue of time scales. Confusion can arise because temperature varies so much from year to year and even decade to decade. He writes:
For instance, over the last 100 years, there was an overall warming which was stronger toward the end of the 20th Century. This is why some say “warming is accelerating.” But if we look at a shorter, more recent period of time, say since the record warm year of 1998, one could say that it has cooled in the last 10-12 years. But, as I mentioned above, neither of these can tell us anything about whether warming is happening “now”, or will happen in the future.
So the problem with focusing on what has happened over any 10- or 15-year period is that climate change doesn’t necessarily occur over that kind of time frame. Global surface temperatures can fluctuate a lot from year to year. For example, the Mount Pinatubo volcanic eruption in 1991 sent a lot of aerosols into the atmosphere, reflecting sunlight away from the Earth and lowering temperatures over the next couple of years. Fluctuations can also occur in El Niño years, when the Pacific Ocean releases a lot of heat, and likewise during the opposite event called La Niña, which cools the climate.
The following chart (click to enlarge) from the University of Alabama at Huntsville shows atmospheric temperature variations from 1978 to 2012. The tallest red spike corresponds to the 1998 El Niño year, so the temperature shot up that year. Likewise, 2008 was a La Niña year, so you see a pronounced negative blue spike for that year.
Naturally, these kinds of year-to-year variations will affect the averages on a short-term basis. Suppose you arbitrarily selected 1998 as your starting year and 2008 as your ending year and tried to generate a trend? You could draw the incorrect conclusion that the global troposphere temperature declined almost a whole degree Celsius over that period. In fact, one advocate claims that that is exactly what some climate-change critics do, drawing a line “from the warmest year (the high peak) to the lowest La Niña they can find,” thus falsely giving “the impression that an ordinary La Niña is actually a cooling trend.”
However, if you look at that chart overall, it’s pretty clear that the trend is from blue to red on a scale of decades — the trend is warming.
The following “escalator” graphic from Skeptical Science helps you to see how you could get the wrong impression about temperature changes by focusing on shorter-term apparent pauses and miss the longer-term trend.
On top of all this, the focus on short-term average temperature measurements misses a much larger point, according to environmental scientist Dana Nuccitelli: the continuing increase in the Earth’s total heat content. Nuccitelli writes that, while we live on the planet’s surface and within its atmosphere, “the land and atmosphere are just a small fraction of the Earth’s climate.” To understand how the climate is actually changing, you have to understand what is happening in the ocean and how the ocean and the atmosphere are exchanging heat. The total heat content is shown in the following trend chart, where you can see that the total heat content for the earth system is continuing to grow, especially in the ocean.
Why So Much Controversy?
Why is it so hard for people to figure out human-caused global warming? Part of the problem is that it’s just so complex a topic. I’ve studied the science recently in a university course, and I struggle with it.
I disagree with activists who try to paint climate-change critics as “anti-science.” For the most part, critics don’t seem to be against science in itself. And I don’t think it really works to paint the critics as “denialists,” as if questioning human-caused global warming were somehow equivalent to denying the Nazi holocaust.
What I think is that the climate-change controversy is more of a social controversy than a scientific controversy (see my previous piece, “All This Wrangling Over Climate Change – What’s Up With That?“) Most people just don’t have the time and expertise to figure out complex topics, so they look to trusted sources to help them understand. As my dad used to say, “I don’t know what to think until I’ve read the editorial page.”
People tend to line up on one side or the other of this controversy according to political and ideological leanings and depending on who they trust and who they like to listen to.
The resistance to the narrative of human-caused global warming tends to come from people who are worried that reducing greenhouse gas emissions will cause economic harm. Why put a drag on development for a danger with so much uncertainty attached to it? That seems to be Monckton’s point of view, when he says:
The Earth has not been warming at anything like the predicted rate and is not now at all likely to do so; and, in any event, even if the climate-extremists’ predictions were right, it would be at least an order of magnitude more cost-effective to wait and adapt in a focused way to any adverse consequences of man-made “global warming” than it would be to tax, trade, regulate, reduce, or replace CO2 today.
Global warming is a high-stakes controversy — and when the stakes are high, people fight hard to protect their positions.
[Editor's Note: This article is the first in a series examining key arguments about climate change. The two other articles examine whether Arctic sea ice is really shrinking and whether sea levels are really rising.]