Three key questions need to be asked in relation to climate change:

  1. is global warming (climate change) happening?
  2. is the human race primarily responsible for recent global warming?
  3. what effects might we expect, as a result of global warming?

This and the following two pages address these questions. The first question we consider is: what evidence is there that global warming is actually happening?

The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by two United Nations organizations, the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess the "risk of human-induced climate change". IPCC reports are widely cited in almost any debate related to climate change. National and international responses to climate change generally regard the UN climate panel as authoritative. A small but vocal minority (nearly 1.5%) of the scientists involved with the report have accused the IPCC of bias.

Note: the first part of the new, fourth U.N Intergovernmental Panel on Climate Change (IPCC) report came out on the 2nd February 2007 (IPCC 2007). This report states that many of the changes quoted below, and in the next two pages are now: more likely to be happening, to happen in future, or to be due to human activites, where appropriate.

For the moment I will leave most of the following data as it is. But where "Update" appears in the following pages, this refers to newer data. But please note that some of the following climate change pages now present the data in a way that is slightly more conservative than the emerging consensus.

Evidence for global warming

There are a number of sources of data that provide evidence for global warming. Inevitably when dealing with such a complex system and slow changes (relative to human lifespan), any data only indicates probabilities rather than certainties. Much of the following is contained within the Third Assessment Report (TAR) of the IPCC (TAR summary for policy makers, 2001).

The surface thermometer record

The surface thermometer record has been taken at thousands of points around the world for well over a century. By combining these data, it has been found that the Earth's average surface temperature has increased by 0.6oC (0.4-0.8oC) during the 20th century. The 1990s were the warmest decade since records began. This data set is good, as it directly measures temperature using simple, robust technology with less room for error than other more complex means. Figure 1 shows the temperature anomaly in global average surface temperatures from 1856 to 2005.

Update: the warming trend for the last 50 years is 0.13oC per decade, nearly twice that for the previous 100 years. The total global average temperature increase from 1850-2005 is now 0.76oC.

global average surface temperature anomaly
Figure 1. Global average surface temperature anomalies from 1856 to 2005, plotted relative to the 1961 to 1990 average temperature (the zero line). The red line represents five year averages (adapted from Instrumental Temperature Record)

Anomalies are often used, rather than absolute measures, as the nature of the data frequently shows changes in quantities better than absolute. The anomaly is plotted against some reference figure, in this case the 1961 to 1990 average temperature. As shown, global average surface temperatures have been higher than this average for at least 20 years. Also, since 1979, land temperatures have increased about twice as fast as ocean temperatures, 0.25oC per decade against 0.13oC per decade respectively (Smith, 2005). Update: 11 of the last 12 years (1995-2006) were among the warmest on the surface temperature record.

The glacier record

Glaciers are recognized as one of the most sensitive indicators of climate change, advancing substantially during climate cooling (e.g. the Little Ice Age) and retreating during climate warming, over moderate time scales. Glacier lengths have been measured for hundreds of years. Of the 36 glaciers measured between 1860 and 1900, 35 retreated and 1 advanced. Of the 144 measured between 1900 and 1980, 142 retreated and 2 advanced. Figure 2 shows the average shrinkage of measured glaciers since around 1800.

change in average glacier length
Figure 2. Change in average glacier length over 300 years, relative to the length measured in 1950 (Dessler and Parson, 2006)

The Greenland ice cap is a massive store of frozen fresh water on land. As shown in figure 3, the year 2002 saw the maximum yearly melt seen so far in the observational record. Surface melting extended up to 6,560 feet (2,000 m) in elevation in the northeast portion of the island, where temperatures normally are too cold for melting to occur. In addition, the total melt area covered 265,000 square miles (686,350 km2), representing a 16% increase above the maximum melt area measured in the past 24 years (Earthobservatory, NASA).

melting of the Greenland ice sheet
Figure 3. Extent of summer melting of the Greenland ice sheet in 1992 and 2002 (from earthobservatory NASA, vanishing ice)

The pattern of glacier melting closely matches the pattern of global average temperature changes. The simplest cause of melting is warming climate, but as the size of glacier is due to a balance between snow accumulation and melting, decreasing snow fall could also lead to the same effect. However, large reductions in snowfall would be required, and the shrinkage of glaciers is a global phenomenon that has occurred steadily over the last century. Most glaciologists therefore deem global warming to be by far the most likely cause of glacial melting.

Update: losses from the ice sheets of Greenland and Antarctica have very likely contributed to sea level rise over 1993 to 2003 (IPCC 2007).

Sea level changes

Sea levels may rise due to three reasons:

  1. water expands when warmed, so the seas also expand as global temperatures increase
  2. melting ice (glaciers and other land ice)
  3. loss of other water stored on land, such as lakes, aquifers etc

Data from sea level gauges show that there has been an average increase in sea level of 15cm over the 20th century. It seems likely that thermal expansion accounts for about half of this increase.

Update: global average sea level rose at an average rate of 1.8mm (1.3-2.3mm) per year over 1961 to 2003. The rate was faster over 1993 to 2003, about 3.1mm (2.4-3.8mm) per year (IPCC 2007).

Sea ice coverage

The average arctic sea ice area has also shown a long-term decrease over the last few decades. Figure 4 shows the annually averaged anomalies in sea ice area from 1973 to 2000. There has been an average rate of decrease in sea ice area of 2.8% per decade, with larger decreases in summer of 7.4% (5.0-9.8%) per decade.

change in average arctic sea ice area
Figure 4. Change in average annual arctic sea ice area between 1973 and 2000, relative to the 1973 to 1996 average area (Dessler and Parson, 2006)

There has also been a thinning of sea ice. Over the last several decades arctic sea ice has become about 40% thinner. This data is from the arctic only. The antarctic is more complex, having substantial land ice as well as sea ice masses. It seems that the data from the South Polar region is less easily interpreted than that from the North.

Climate Proxies

These are used to estimate past climatic variations. They include:

  1. Ice cores (from glaciers): the chemistry of the ice and trapped air bubbles can be used to provide information about the climate in the past. By drilling deeply, it is possible to obtain data for more than 400,000 years ago.
  2. Ocean sediments: the proportion of different species in the sediments, as determined from their microscopic skeletons, provides information on surface water temperatures in the past. Chemical compositions of the skeletons are also measured.
  3. Underground boreholes: temperatures measured at different depths underground can provide data on land surface temperatures over the last several hundred years.

There are also other methods used, such as satellite temperature measurements. What conclusions have been reached from these sources, in answer to the question "is the Earth warming?" The table below summarises the key findings, and clearly shows that the Earth is warming, quite rapidly.

Type of data Direction of change Size of change
direct surface air temperature warming average temperature increased 0.6oC over 20th century, with about half of this occurring since 1980
glaciers warming receding on average for at least 200 years. This implies a warming of 0.6oC per century
sea-level change warming sea-level rose by about 15cm over the 20th century. About half of this was probably due to thermal expansion.
sea ice warming the area of Arctic sea ice in spring and summer has decreased by up to 15% over the last 50 years. Average thickness has also decreased by 40% over the same period.
ocean temperature warming the top 300m of ocean has warmed by 0.18oC over the last 50 years
climate proxies warming combined data from many proxies shows gradual Northern hemisphere cooling from 1000AD until the 1800s. Since then there has been strong warming.
satellite temperature measurements warming satellite instruments show warming of 0.06-0.26oC per decade
Table 1. A summary of indicators of changes in the Earth's temperature (table after Dessler and Parson (2006), p65)


The following extracts come from the IPCC Third Assessment Report summary for policymakers (IPCC, 2001):

"The global average surface temperature (the average of near surface air temperature over land, and sea surface temperature) has increased since 1861. Over the 20th century the increase has been 0.6 ± 0.2oC."
"Globally, it is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record, since 1861."
"New analyses of proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years." Figure 5 shows changes in the average surface temperature of the Earth over the last 1000 years.
Variations of the Earth's surface temperature over the last 1000 years
Figure 5. Variations of the Earth's surface temperature over the last millennium. Data from thermometers (red), tree rings, corals, ice cores and historical records (blue). The year by year (blue curve) and 50 year average (black curve) variations of the average surface temperature of the Northern Hemisphere for the past 1000 years have been reconstructed from proxy data calibrated against thermometer data. Note: there is quite large uncertainty in proxy data for past climates, of the order of 0.5oC (adapted from IPCC, 2001)
"Satellite data show that there are very likely to have been decreases of about 10% in the extent of snow cover since the late 1960s, and ground-based observations show that there is very likely to have been a reduction of about two weeks in the annual duration of lake and river ice cover in the mid and high latitudes of the Northern Hemisphere, over the 20th century."

Update: "More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics"..."frequency of heavy precipitation events has increased over most land areas, consistent with warming and observed increases of atmospheric water vapour"..."flow speed has increased for some Greenland and Antarctic outlet glaciers, which drain ice from the interior of the ice sheets" (IPCC 2007).

There is now strong evidence of a rapidly warming world, which has been reinforced in the latest IPCC report. The next page (climate change 2) addresses the question: "is this recent global warming primarily the human race's fault?"

References and links

  1. The surface thermometer record
  2. The glacier record
  3. Sea level changes
  4. Sea ice coverage
  5. Climate proxies
  6. Conclusions
  7. References and links