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)
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.
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). Eleven of the twelve years between 1995 and 2006 were the warmest since 1850, when the instrumental record began.
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. 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. 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.
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.
 IPCC, Fourth Assessment Report, Working Group I, Summary for Policy Makers, p. 5