Research Features

Earth's Temperature Tracker, Page 2

The Data and the Details

Some nagging questions remained for Hansen and his colleagues. Citing issues such as stations located too close to paved surfaces, stations located in urban areas that are known to be warmer than rural regions, and stations located in developing nations where data collection methods may be unreliable, critics argued that any of these problems could throw off an individual station's temperature readings. Don't such concerns cast a shadow of doubt on the NOAA weather station data?

Initially, perhaps, but not after the data have been carefully tested in several ways. First, Hansen's team (and others) finds good agreement of the weather station data with "proxy" data sets that are sensitive to surface temperature changes—such as the rate at which glaciers are receding, or subsurface temperature measurements in boreholes drilled down into the ground. (Scientists can infer surface temperature change from underground temperatures based on equations that describe how heat diffuses through the ground over time.) The results in thousands of remote locations around the world agree well with the surface temperature measurements.

Second, Hansen's team "cleans" the weather station data by finding and filtering out flawed data entries. Specifically, they apply a computer algorithm that checks each data point for temperature readings that are very significantly higher or lower than average for a given location at that time of year. Whenever such an anomaly is flagged, the algorithm compares those data to data from nearby stations to see if they show a similar anomaly. If so, then the data in question are kept; if not, or if there are no nearby stations for comparison, then the data are thrown away.

Graph of temperature for Linyi, China showing an outlier.

Bad data are cleaned from the NASA global temperature record by first looking for outliers: months when the temperature at a station is much higher or lower than the average for that time of year. The monthly temperature record for Linyi, China, in 1932 (red dots; June data is missing) shows that September was 5.3° C warmer than average. The unusual data point was compared to nearby stations. Since some of those stations were also exceptionally warm, the data point was retained. If nearby stations do not confirm the anomaly, the team does not use the data. (Graph by Robert Simmon, based on data from the GISS Surface Temperature Analysis Station Data.)

His team also modifies the data from stations located in densely populated areas by removing the long-term bias of these "urban heat islands." The team uses satellite data to determine if a given station is in an urban or near-urban location. If so, then the team uses the nearest rural stations to determine the long-term trend at the urban site. If there are no rural neighbors, then Hansen's team throws out the urban station data.

Map of urban areas and weather stations in the United States.

Weather stations are screened for potential bias from urban heat islands by comparing station locations with maps of urbanization. Measurements from nearby stations in rural areas (gray) are used to correct urban station data for warming due to the heat island effect. If no rural neighbors are available for comparison, data from urban (dark blue) and peri-urban (blue) stations are left out of the global average calculation. (Map by Robert Simmon, based on data from NOAA.)

One lesson to be learned here is weather science and climate science are quite different: weather is concerned with what conditions are like at a given location and time, whereas climate is concerned with what conditions are like over large regions, or over the entire globe, and for a long period of time. That explains why climate scientists are not as interested in any given reading for an individual station as they are in 5-year and 10-year blocks of time for the entire planet.

Hansen acknowledged there may be flaws in the weather station data. "But that doesn't mean you give up on the science, and that you can't draw valid conclusions about the nature of Earth's temperature change," he asserted.

From A Dimmer Past to a Brighter Future?

Of greater concern to Hansen than global warming skeptics is the problem of global warming itself. If greenhouse gases are to blame then why did Earth's average temperature cool from 1940-1970? And why has the rate of global warming accelerated since 1978? Hansen's answers to these questions brought him full circle to where he began his investigation more than 40 years ago.

"I think the cooling that Earth experienced through the middle of the twentieth century was due in part to natural variability," he said. "But there's another factor made by humans which probably contributed, and could even be the dominant cause: aerosols."

Photograph of a smokestack

Pollution from factories, cars, airplanes, home furnaces, and power plants form aerosols—tiny particles suspended in the air. These particles reflect and absorb sunlight, slightly cooling the Earth's surface. (Photograph © 2007 Señor Codo.)

In addition to greenhouse gas emissions, human emissions of particulate matter are another significant influence on global temperature. But whereas greenhouse gases force the climate system in the warming direction, aerosols force the system in the cooling direction because the airborne particles scatter and absorb incoming sunlight. "Both greenhouse gases and aerosols are created by burning fossil fuels," Hansen said, "but the aerosol effect is complicated because aerosols are distributed inhomogeneously [unevenly] while greenhouse gases are almost uniformly spaced. So you can measure greenhouse gas abundance at one place, but aerosols require measurements at many places to understand their abundance."

After World War II, the industrial economies of Europe and the United States were revving up to a level of productivity the world had never seen before. To power this large-scale expansion of industry, Europeans and Americans burned an enormous quantity of fossil fuels (coal, oil, and natural gas). In addition to carbon dioxide, burning fossil fuel produces particulate matter—including soot and light-colored sulfate aerosols. Hansen suspects the relatively sudden, massive output of aerosols from industries and power plants contributed to the global cooling trend from 1940-1970.

Graph of sulfur concentration in Greenland ice from 1880 to 2000

Sulfur trapped in the Greenland Ice Sheet records the presence of reflective sulfate aerosols downwind of the United States and Canada. Emissions of the pollutants that form sulfate aerosols rose sharply in the United States and Europe during and after World War II. This rise may be responsible for the Northern Hemisphere cooling from 1940-1970. By the 1980s, oil embargos and environmental controls had reduced sulfate pollution in North America, but carbon dioxide continued to build up in the atmosphere. (Graph by Robert Simmon, based on data from McConnell et al., NOAA/NCDC Paleoclimatology Program.)

"That's my suggestion, though it's still not proven," he said. "There is a nice record of sulfates in Greenland ice cores that shows this type of particle was peaking in the atmosphere around 1970. And then the ice core record shows a rapid decline in sulfates, right about the time nations began regulating their emission." (Sulfates cause acid rain and other health and environmental problems.)

In 2007, Michael Mischenko, of NASA GISS, published a paper in the journal Science in which he reported tropospheric aerosols have indeed declined slightly over the last 30 years. The net effect is that more sunlight passes through the atmosphere, slightly brightening the surface. This increased exposure to sunlight could partially account for the increase in surface temperature that Mischenko and Hansen observed over the same time span.

Graph of aerosol optical thickness from 1981 to 2005

Satellite observations of aerosol optical thickness (how greatly aerosols reduce the intensity of sunlight reaching the surface) show that aerosol concentrations have decreased since 1991 (green line). Prior to that, they had been rising slightly (blue line). In addition to the long-term trends of human-made aerosols, the graph shows the occurrence of large volcanic eruptions like El Chichón in 1982 and Mount Pinatubo in 1991. These natural events produce large spikes in aerosol concentrations, but their impact is short-lived. (Graph adapted from Mishchenko et al., 2007)

Over the course of the twentieth century, Hansen and other climate scientists estimate aerosols may have offset global warming by as much as 50 percent by reducing the amount of sunlight reaching the surface. Scientists call this phenomenon "global dimming," although the change was too gradual and too slight to be perceived by the human eye. (Aerosols' dimming potential has been observed, of course, after dramatic events like the Agung Volcano eruption that Hansen noticed during the lunar eclipse of December 1963.)

Hansen describes the global dimming effect of human-emitted aerosols as a "Faustian bargain"—a deal with the devil. "Eventually you get to a point where you don't want aerosols in the atmosphere because they're harmful to human health, harmful to agriculture, and harmful to natural resources," he stated. "So in the U.S. and much of Europe, we've been reducing aerosol emissions."

But we haven't seen a corresponding reduction in greenhouse gas emissions. Indeed, humans' use of fossil fuels rose rapidly (about 5 percent per year) from the period after World War II until 1973. After the oil embargo and price shock of oil in 1973, annual average consumption continued to increase, but at a slower pace (between 1.5 and 2 percent per year). A byproduct of that rising fossil fuel consumption has been a corresponding rise in carbon dioxide emission. Because greenhouse gases reside in the atmosphere for decades, while aerosols usually wash out over a span of days to weeks, the warming influence of greenhouse gases gradually won out.

"For much of the twentieth century, both types of human emissions were on nearly equal footing, and aerosols were able to compete with greenhouse gases," Hansen said. But that balance has tilted increasingly in favor of greenhouse gases in the last 30 years. Today, Hansen's team estimates the human forcing from greenhouse gases to be about 3 watts per square meter (warming) and the forcing from aerosols to be about minus 1.5 watts per square meter (cooling). Hansen sees these trends as very likely to lead to what he calls "dangerous human interference" with the climate system.

"I think action [to reduce greenhouse gas emissions] is needed urgently, because we are on the precipice of a climate system 'tipping point'," Hansen concluded. "I believe the evidence shows with reasonable clarity that the level of additional global warming that would put us into dangerous territory is at most 1°C."

Map of 2001 to 2006 global temperature anomaly

This map shows the difference in surface temperature in 2006 compared to the average from 1951 to 1980. Most of the globe is anomalously warm, with the greatest temperature increases in the Arctic Ocean, Antarctic Peninsula, and central Asia. NASA's effort to track temperature changes will help societies evaluate the consequences of global climate change. (Map based on data from NASA GISS Surface Temperature Analysis.)

If we follow a 'business-as-usual' course, Hansen predicts, then at the end of the twenty-first century we will find a planet that is 2-3°C warmer than today, which is a temperature Earth hasn't experienced since the middle Pliocene Epoch about three million years ago, when sea level was roughly 25 meters higher than it is today.

[Editor's note: The NASA GISS Surface Temperature Analysis site contains additional discussion, sample maps and graphs, and links to the programs used by Hansen's team to process the surface temperature data.]


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Further Reading

Intergovernmental Panel on Climate Change 2007: Climate Change 2007: Climate Change Impacts, Adaptation and Vulnerability Summary for Policymakers, A Report of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.

Intergovernmental Panel on Climate Change 2007: Climate Change 2007: The Physical Science Basis Summary for Policymakers, A Report of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.

Lindsey, Rebecca, ed. 2007: Global Warming Questions & Answers. NASA's Earth Observatory.

Riebeek, Holli 2007: Global Warming. NASA's Earth Observatory.

U.S. Climate Change Science Program 2006: Temperature Trends in the Lower Atmosphere. Accessed April 13, 2007.

U.S. Environmental Protection Agency 2007: Climate Change. Accessed March 22, 2007.

Weart, Spencer 2007: The Discovery of Global Warming. American Institute of Physics.

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