This article is the first in a series of blog pieces inspired by a report responding to claims of those skeptical of climate science, recently completed by the the Columbia Climate Center in collaboration with Deutsche Bank Climate Change Advisors.
Skeptic Claim: Earth is not warming.
Some examples of this claim include:
- “The global climate has warmed over the last 100 years, but not appreciably over the last 50 years…since 1940, weather satellites, tree ring data, and corrected thermometer readings all agree that climate has not warmed as much.” (http://www.sepp.org/, FAQs)
- “Temperatures are not rising: Satellites circling the planet twice a day show that the world has not warmed since 2001.” (Joanne Nova, The Skeptics Handbook, p. 3)
Brief Response:
On the contrary, multiple measures of global temperature, including the average air temperature (air near land surface), the average temperature of the ocean surface, and the average temperature of the lower troposphere (the lowest 5 miles of the atmosphere) have increased over the period in which they have been recorded. This increase constitutes a long-term warming trend. Other observations, such as glacier retreat, decreasing Arctic sea ice, and rising sea levels, are consistent with rising temperatures and further support the conclusion that Earth is warming.
It is important to remember that a long-term warming trend does not mean that each year will be warmer than the last. Local temperatures, which are how we most readily experience climate, may be quite different from global averages. Winters, therefore, will not disappear, and we will continue to experience colder-than-average periods. In addition, the climate system undergoes natural, internal variability (such as the 5-7 year El Niño-Southern Oscillation or the 20-30 year Pacific Decadal Oscillation). A long term warming trend means that over a long period of time, the average temperature of a given area will rise, as will the upper limit of the temperature range.
Longer Response:
There are several independent observations that Earth has warmed since the Industrial Revolution, or about 1850. One of the most important measures of this warming is the global surface temperature, which uses temperature over land and sea surface temperature to characterize the surface temperature of the planet. Average global surface temperatures have risen noticeably in the industrial era: from 1850-1899 to 2001-2005, global average temperatures increased by approximately 0.76˚C (1.37˚F)[1]. Eleven of the twelve years between 1995 and 2006 were the warmest since 1850, when the instrumental record began.[2]
Graph 1 shows the how global average temperature anomalies have changed since 1850. Temperature anomalies reflect the degree to which temperatures differ from a baseline period (in this case, the baseline is 1961-1990). A positive anomaly means that temperatures were higher than during the baseline period; negative anomalies indicate that temperatures were lower.
Graph 2 shows that not only have temperatures risen, but the rate of temperature rise has also increased. Temperatures rose nearly twice as fast in the past 50 years (an estimated 0.13˚C (0.23˚F) per decade) as they did in the past 100 years.[3] This is shown clearly by the increasing slopes of the colored lines: the slopes become steeper in recent decades as Earth’s global mean temperature has risen more quickly.
As explained above, long term trends can and do occur concurrently with shorter-term changes due to variability. In the case of the current warming trend, this means that while we may experience periods of relatively stable or even cooler temperatures, warming continues in the long-term. The difference between this short-term variability and long-term changes has resulted in significant confusion, as illustrated by the claim that there has been no warming since 1998 (see, for example, Bob Carter in The Telegraph).
Indeed, global average temperatures were anomalously high in 1998 because of the unprecedented magnitude of that year’s El Niño event. The relatively lower temperatures in following years therefore represent cooling. However, global surface temperature since 1998 have remained close to the 1998 level and the decade of 2000 to 2009 was the warmest in the instrumental record. The map below shows how temperatures in the decade 2000-2009 have changed as compared to the global average between 1951 and 1980: darker red areas show higher temperature increases.[4] The map indicates how temperatures evolve differently in different regions. The US warmed less than most of Western Europe or Central Africa, while Siberia has cooled. Regional variability can therefore also be a source of confusion.
The record cold and snowy winter of 2009-2010 in the eastern U.S. demonstrates how confusing variability over both time and space can be. This “snowpocalypse” was used by some as evidence that Earth is not warming. Senator Jim DeMint (R-NC), for example, remarked that “It’s going to keep snowing in DC until Al Gore cries ‘uncle.’”
The record-breaking snow, however, did not reflect temperatures of the entire Northern hemisphere nor those of the entire planet: while the Eastern Seaboard saw unusually snowy conditions, parts of the Arctic and Canada experienced unseasonable warmth, causing considerable headaches for the organizers of the Winter Olympics in Vancouver. In fact, the global temperature anomalies for January-March 2010 were the fourth warmest on record. Far from being proof that Earth’s climate is not changing, the cold and snowy winter on the East Coast resulted in part from a well-known periodic phenomenon in which extreme negative values of the Arctic Oscillation cause a weaker jet stream and exchange of cold Arctic air with milder mid-latitude air.[5]
Many factors affect climate, including natural processes, like changes in solar activity, and human activities, such as increases in greenhouse gases in the atmosphere. The warming observed in the latter half of the 20th century is consistent with greenhouse gas emissions from human activities. In fact, this warming can only be explained by anthropogenic greenhouse gas emissions, since there are no known natural drivers of climate that can explain the observed warming. This conclusion has been concretely expressed the U.S. National Academy of Science, which found in 2010 that “climate change is occurring, is caused largely by human activities, and poses significant risks for a broad range of human and natural systems.”
Conclusion: The surface temperature unequivocally shows that Earth is warming. Periods of stable average temperatures, and periods of colder than average temperatures, are consistent with the natural variability of Earth’s climate, and do not undermine the long-term warming trend that has been clearly observed. The Environmental Protection Agency, the Intergovernmental Panel on Climate Change, and the U.S. National Academy of Sciences assert that this warming trend can only be explained by anthropogenic greenhouse gases.
[1] IPCC, Fourth Assessment Report, Working Group I, Summary for Policy Makers, p. 5
[2] ibid
[3] ibid
[4] For more information, also see: NASA GISS 2010, WMO 2010, NOAA NCDC 2010b, Hansen et al. 2010, Arndt et al. 2010
[5] For more information, see this article from The New Republic.
“this warming can only be explained by anthropogenic greenhouse gas emissions, since there are no known natural drivers of climate that can explain the observed warming.”
In order to say that you need to ignore the change in ENSO that is apparent in changing atmospheric pressure over the period of record. Atmospheric pressure determines wind strength and is tied in with changes in cloud cover. Atmospheric pressure is not affected by urban warming phenomena. Better look a little more closely. What you say puts peoples living standards and the possibility of economic growth for non industrialized world at stake. Be careful.
Like Nelson, you choose to look with the blind eye.
Erl, you’re just plain wrong. Open your own eyes to the scientific evidence, which encompasses multiple lines beyond your limited focus.
To name just a few, how does ENSO explain away the empirical evidence of accelerating ocean acidification, glacier melt, shifting ranges for both plants and animal species, and melting permafrost?
Your last sentence is quite revealing: by conflating the scientific evidence with potential policy responses, you show you actually care most about money — and your own comfort, which you erroneously perceive to be threatened — than you do about the world you are creating for your children.
i’d like to see one example of this response working. just one! indeed, i’d like to hear from someone who has since denounced denialism because of such responses. (please, please send me examples).
yes, erl’s view of the facts is influenced by his concern about policy impact and costs. and methuselah’s is influenced by anxiety about his children. neither of these consequentialist approaches are useful for predicting climate.
although i have a strong background in science, i’m just a lawyer, not a scientist, but have been observing the controversy over global warming for a long time. here is a summary of my impressions so far.
1. proposition: global temperatures are rising. there doesn’t seem to be any serious, evidence-based controversy over this. it may be possible to interpret data as reflecting short-term fluctuations rather than long-term trends, but the data seems to show rising temperatures over the decades that data has been collected. that doesn’t mean lay people will agree about this. but informed, reasonably objective scientists seem to be largely in consensus on this point.
2. proposition: man has caused global warming. in my view this is a credible theory with many uncertainties. the cause of global warming is not directly observable, and we know there have been massive climate changes in earth’s natural history. in my elementary school science classes in the 1960’s, our science book texts taught us that we were at the end of an ice age. if man’s activity is warming the planet, i don’t see how the portion caused by man and the portion that would have occurred but for man’s activity have been or can be accounted for separately.
3. proposition: if man caused global warming, man can reverse at least the portion of global warming he caused. this proposition seems to me to be difficult to demonstrate. first, it seems wrong to assume that if man caused something that he’s likely to be able to reverse it. if i knock a glass off a table and it spills its contents and breaks, i cannot reverse it. the burning of large reserves of fossil fuel seem to me to be highly entropic effects that may not be reversible. put differently, if i were to be convinced that man could reverse global warming, i would not consider it evidence that man has been a substantial contributing cause.
4. if man can reverse global warming, then man should reverse it. this seems to me the least likely and provable of the propositions. first, this is a policy question, not a scientific one, even though it should be considered in light of the best science available. moreover, as with all policy questions, we need an analysis of costs and benefits, discounted for time-value (the time-value of money). to make a rational decision about what to do, we have to compare the best information about the present value of all the future costs of reversible climate change (in light of the best science) with the present value of all future costs to reverse that change. if we’re rational, we will do whichever costs less. it’s also useful to observe that the present value of future costs is determined by a converging function, which means that very high costs that continue indefinitely into the future have a limited (not unlimited) present value. it seems to me that despite massive data supporting trend in increased temperature, there is inadequate data regarding the reversibility of that trend, what it will cost, and the value of benefits, to have a rational policy debate about costs and benefits. that, in my view, is why almost anyone who discusses this topic with any passion, on either side of the debate, looks more like a long-faced preacher than a rational actor. because to make a strong case on either side, you have to appeal to things that aren’t rational, like the supposed well-being of children not yet in existence or how damn cold it was in nebraska last year. in the absence of adequate information about costs and benefits, a rational person should not act. that is why, for me, inaction about climate change is the only sensible response right now. as the discussion becomes better informed with costs and benefits, that may change. the fact that it might then be too late is not a convincing reason to act now without better information.