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Answers to Frequently Asked Questions
Is the Earth Getting Warmer?
|Have surface temperatures
Is the observed temperature rise due to urban heat islands?
Is the observed temperature rise a artefact of changes in coverage?
Do satellite data show that the earth is not warming?
Are the mountain glaciers melting?
Is the Antarctic warming?
Is the Arctic warming?
Is Arctic ice melting?
Is the permafrost thawing?
Are the oceans warming?
|Have surface temperatures risen?|
|All the world's meteorological and climatic centres agree
that global surface temperatures have risen by about 0.6ºC over the past 100 years. For
example, here are some reports from the US National
Climate Data Center, NASA's Goddard Institute, the
UK Meteorological Office,
and the World
The chart on the right shows average surface temperatures since as far back as can be reliably assessed (provided by the UK Meteorological Office).
|Is the observed temperature rise due to urban heat islands?|
|Some weather stations are located in cities, and it is
possible that changes in these cities may make the recorded temperature rise faster than
the background temperature. For this reason, data from stations within 'urban heat islands' are
care detrended, and their trends replaced by average trends from nearby
rural stations. In other words, trends from urban stations are not used
to calculate global temperature trends. A description of this
adjustment is provided in this article
from NASA's Earth Observatory. For a more detailed explanation, see Hansen
et al, 2001. When the trends from urban stations are removed, the increase in
temperatures over the past century is lower, but it is only lower by around 0.05ºC (as
reported on Page
39 of the report from the US National Academy of Science.
The major question about UHI is: have the stations affected by UHI been correctly identified. Several lines of evidence suggest that they have, and that the corrected global temperature trends are broadly accurate. These are as follows:
There are two reasons why the effect is so small. Firstly, only 27% of weather stations are located in urban centres, according the the Global Climate Observing System. The rest are either in small towns (19%) or rural (54%).
Secondly, although large urban heat islands can sometimes be several degrees centigrade warmer than their surroundings, most of them have been warmer than their surroundings since records began. In addition, in some islands the population density has actually decreased, and in others modern urban development has had a lower impact on temperatures. The overall effect is that the rate at which the temperature of heat islands has increased is not much greater than the background rate. A recent study suggests that even this small effect is an overestimate. A recent paper (Peterson; 2003) reporting that "Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures."
For more information on urban heat islands, see this Wikipedia item. See also 'Surface and Satellite Temperature Records, and the Influence of Urban Heat Islands'.
|Is the observed temperature rise an artefact of changes in coverage?|
|The measured increase in the Earth's surface temperature
during the 20th century is based upon thermometer measurements, which become increasingly
incomplete further back in time. For example, at the beginning of the 20th century,
thermometer measurements covered only 20 percent of the Earth's surface, compared to more
than 87 percent in 1987. It has been argued that the gradual increase in coverage during
the 20th century introduced an artificial warming trend into the temperature record, which
accounts for most or all of the 20th century's measured warming. A study
from Lawrence Livermore simulated the effects of changing coverage using 16 different
warming scenarios derived from climate models. The study found no evidence to support the
hypothesis that incomplete observational data has caused us to overestimate the true
|Do satellite data show that the earth is not warming?|
|Measurements using NASA's
Pathfinder show that, from 1981–1998 (the last year of data
currently available), the temperature of the Earth's land surface
increased at a
rate of 0.43ºC (0.77ºF) per decade. However, the Pathfinder satellite
cannot measure temperatures over water, or of ice-covered land. In
addition, things like clouds, volcanic eruptions, and other factors give
false readings of land temperatures, and must be factored out to make the
skin temperature data more accurate.
The Microwave Sounding Unit (MSU) aboard the TIROS satellites has also been used to measure the earth's temperature. The MSU cannot provide a record of the surface temperature, but it can provide data on the temperature of the atmosphere above the surface. Unfortunately, there have been a series of satellites, and it has proved to be very difficult to obtain a consistent, long term record as old satellites are retired and replaced by new ones. In addition, the temperature measured by these satellites is critically dependent on their exact position in the sky. However, this information was not recorded, and so it must be estimated using various different techniques.
Several different research teams have attempted to reconstruct atmospheric temperatures in the mid-lower atmosphere (MSU channel 2), with varying results. There are currently two reconstructions that are generally considered to be potentially accurate. The most well known is that of Christy and Spencer (UAH), first published in 1996. More recently, in 2003, Mears and Wentz (RSS) have analysed the data using a different technique. Although both techniques are broadly in line with predictions from climate models (see Hansen et al 2002 and Santer et al 2003), the RSS analysis shows more warming. It is not currently known which, if either, of these analyses is correct.
The satellite data are further complicated by the fact that they measure a broad band of the atmosphere, tens of kilometres thick. Temperatures changes over such a broad band are expected to differ from temperature changes at the surface - most notably, the upper atmosphere (the stratosphere) is cooling as a result of ozone deletion and the increased greenhouse effect. Two different approaches have been used to try to focus the MSU2 readings to report the temperature changes only in the thin band of atmosphere close to the earth's surface (the lower troposphere). Although until recently they differed, recent improvements in the technique employed by Christy and Spencer have, for the most part, removed the differences with the technique of Fu et al. Both techniques now show that warming in the lower troposphere is similar to that observed at the surface, although differences still depending on whether the technique is applied to the UAH or RSS analysis of MSU2.
Other assessments of atmospheric temperature support satellite observations of a warming troposphere and cooling stratosphere. Recent corrections to data from weather balloons (radiosondes) confirm the satellite data (see Sherwood et al, 2005).Furthermore, the height of the troposphere is increasing and the height of the thermosphere is shrinking (and its density decreasing), both of which are in line with expectations that an increasing greenhouse effect will warm the troposphere and cool the stratosphere.
For a good overview of the recent developments in the measurement of temperatures in the lower atmosphere, see et tu LT?
|Are the mountain glaciers melting?|
|The US National Snow and Ice Data Center has collated all the
available small mountain glacier mass-balance data. Their findings show that, not only
are these glaciers melting, but the rate of melting is accelerating. The NSIDC FAQ file
comments "Since the early twentieth century, with few exceptions, glaciers around
the world have been retreating at unprecedented rates". For the
scientific analysis, see Dyurgerov and Meier,
PNAS 2000. In 2002, joint NASA
and United States Geological Survey study has revealed that "The great majority
of the worlds glaciers appear to be declining at rates equal to or greater than
Glaciers can retreat for a number of reasons, besides increased temperature. One of the most important is decreased rainfall. However, for mid-latitude glaciers, it is temperature that has the dominant effect. It is therefore possible to use the changes in these glaciers over time as a measure of temperature, as was done by Oerlemans, 2005. A description of this study is provided by RealClimate.It found that the temperature reconstructed by glacier retreat since 1600 was similar to that reconstructed using other proxy measures of temperature over the past millennium.
|Is the Antarctic warming?|
|According to the British Antarctic Survey Position Statement, the Antarctic as a whole is not warming
areas of the Antarctic are warming, notably the Wordie Ice Shelf and the northern part of
the Larsen Ice Shelf, whereas the centre appears to be cooling. Climate
models in general suggest that Antarctica will take longer to warm than
the Arctic, primarily because the Antarctic is surrounded by oceans, which
tend to absorb excess heat. A detailed analysis has shown that the cooling of central Antarctica is
probably a result of ozone depletion, which has affected a pattern of
atmospheric circulation called the Southern Annular Mode.
The Antarctic Ocean is warming faster than expected.
In recent years, there has probably been a small increase in Antarctic sea ice, although the fringes appear to be melting faster. Evidence from ice cores, however, indicates that there has been an overall decline in Antarctic sea ice since the 1950s (Curran et al, 2003). T Because Antarctica is so cold, global warming can actually cause the ice sheet to thicken, because it increases the amount of humidity in the southern ocean and thereby increases snowfall.
An expert report in 2001 has found that the ice pack has a low (5%) chance of collapsing completely in the next 100 years, but predicts dramatic consequences for sea level if it does. Since then, studies have shown that the ice may be more vulnerable than previously thought. The loss of floating ice, such as the recent collapse of the Larsen ice shelf, can accelerate the flow towards the sea of the ice once held behind it. The remaining sections of the Larsen Ice Shelf have thinned by 18 m since the collapse.
The loss of ice is not connected with volcanic activity, according to this article from USA Today: "There is evidence of a volcano under the West Antarctic Ice Sheet. But, says Philip Kyle of the New Mexico Institute of Mining & Technology, even if there is an erupting volcano under the ice, it's not going to melt the ice sheet. Essentially it comes down to the fact that there is a lot of ice and even a large volcano is not going to melt much of it."
|Is the Arctic warming?|
changes in Arctic temperature are a magnified version of global changes,
and the Arctic is currently warming at a faster rate than the global
average. The figure on the right shows the average temperature in the most comprehensive
dataset produced to date (Johannessen
et al, 2004). Johannessen et al explain that, whereas
the most recent warming is due to changes in greenhouse gases (probably
connected with changes in the Arctic Oscillation), the
warming in the early part of the century was probably mostly natural. Bengtsson
et al (2004) found that the early twentieth century warming was
probably a result of the influx of warmer water into the Barents Sea,
resulting from a short-lived change in wind patterns.
The National Science Foundation examined the station data and the 400-year paleoclimatic record. They found that current Arctic temperatures are the highest in paleoclimatic record, and that the increase in temperatures matches model predictions (see also Overpeck et al, 1997). The US Polar Science Center provides a detailed review of the warming, and shows that the winter warming trend since 1979 has been 2°C per decade. According to NASA, the relatively greater increase in winter temperatures is the predicted result of greenhouse gas accumulation.
The reality of these changes even shows up in the composition of bird feathers. Because the arctic has been warming for the past 100 years, arctic guillemots have been able to spend the winters further north. This change in their habits can be tracked by analysing the 13C composition of feathers from museum specimens, as discussed in an article (Section V) originally published in the NY Times. (See also: Alaskan Warming and PDO)
|Is Arctic ice melting?|
data show that Arctic ice is decreasing at the rate 9% per decade. Less sensitive methods extend
the record back to 1950, and these show that with six of the 10 years of minimum arctic
ice extent have occurred since 1990, according to the US
Snow and Ice Data Center.
A more sensitive measure of changes in Arctic ice is its thickness. The Polar Science Center concludes that "the mean ice draft at the end of the melt season has decreased by about 1.3 m in most of the deep water portion of the Arctic Ocean, from 3.1 m in 19581976 to 1.8 m in the 1990s ... Preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s". These data are corroborated by an analysis of similar data from British submarines, reported in this news article. In Greenland the ice sheet is also rapidly thinning, as measured by aerial mapping over the past decade and, further back, using land-based altimeters.
The cause of these changes is not known for certain. It is probably partly due to changes in surface temperature, but also partly due to changes in the North Atlantic Oscillation (see below).
|Is the permafrost thawing?|
|The Geophysics Institute, University of Alaska, Fairbanks, reports that
"In many areas of both interior Alaska and Siberia, permafrost has warmed to within
one degree Celsius of thawing". The warming of the permafrost may
add to the greenhouse effect, as microbial activity increases and releases
CO2, as explained in this BBC news report.
A graph showing temperature changes at various depths of the Alaskan
permafrost over the past 50 years is provided
|Are the oceans warming?|
surface temperatures are shown in the figure on the right (data
provided by NOAA). On the whole, they show a similar trend to that of
global land surface temperatures.
The oceans are also warming at depth. There are sufficient data from the upper 3,000 m to provide a global estimate. In March 2000, NOAA reported that this layer had warmed on average by 0.06°C over the past 40 years. Most of this warming occurred in the top 300m, which warmed by around 0.3°C.
These data show that heat is entering the ocean from the surface and spreading downwards. The pattern of warming is remarkably similar to predictions from climate models, suggesting that the cause of the warming is primarily greenhouse gases.
Data from even deeper water are rare. Fukusawa et al surveyed a section of deep Pacific water in 1985 and 1999. The results are shown in the figure on the right. They found that warming below 5,000 m amounted to 0.005°C. Although small, this increase in temperature was too great to be explained by geothermal heat from the sea floor.
|Are the corals dying?|
|Around the world, corals are bleaching (dying), according to
a US Government report. This is caused by a combination of an increase
in sea surface temperatures of '0.5 °C a decade, according to Alan Strong of the National
Oceanic and Atmospheric Administration in the US', and increasing CO2 levels (reported in
an article in the New Scientist).
|Is the sea level rising?|
|You can see a graph of changes in global sea level over the
past century here. There are
various estimates of sea level rise over the past century. The differences are mainly a
result of different methods used to adjust for the vertical movements of continental
plates. According to the UK's Permanent Service for Mean Sea Level, the overall rise for the
past century has been 1020 cm. The various
individual estimates are provided by the Colorado Center for Astrodynamics Research.
data reveal that, since 1993, the sea level has risen at a rate of 2.8 mm per year, or
28 cm per century.
For more details on changes in sea levels, see Chapter 11 of the IPCC Third Assessment report. For information on some of the complexities of sea level measurement, as well as other information and suggested references, see What's wrong with 'Still Waiting for Greenhouse'?
|Is the rise in sea level normal?|
|The American Geophysical Union reviewed the data and concluded that "There is a convincing
body of evidence that the sea level rise value of the last 100+ years has not pertained to
the last 2 millennia". Likewise, the US National Climate Data Center states
that "Global mean sea level has been rising at an average rate of 1 to 2 mm/year
over the past 100 years, which is significantly larger than the rate averaged over the
last thousand years".
|Is the North Atlantic Oscillation behaving normally?|
|The NAO (also known as the Arctic Oscillation) is an
atmospheric and ocean cycle similar to El Niño, but located in the northern Atlantic
Ocean. Changes in the NAO alter the climate in Northern Europe. The negative phase brings
higher-than-normal pressure to the polar region and lower pressure to Eurasia and North
America. The positive phase brings the opposite: warmer, wetter weather to northern
regions and dryer conditions to lower latitudes. The NAO has been stuck in the positive
phase for the past 30 years, and its current unusual activity is thought to be partly
responsible for the melting Arctic ice pack (see above). There is
good evidence that the current state of the NAO is a result of climate change, according
to this article from the UK's New Scientist Magazine (for a more technical
Visbeck et al, 2001 and Hoerling
et al, 2002):
"In recent years ... the NAO has been in a period of unprecedented activity. Its flips are bigger, and when the index is averaged over the winter it has usually been positive, often strongly so ... The last time the NAO was routinely positive was between 1900 and 1930. That period also coincided with a spell of planetary warming ... Rodwell <of the UK Meteorological Office> showed that a large, positive winter index was tied to changes in sea surface temperatures off the US coast the previous September ... Tim Osborn of the University of East Anglia recently ran a series of global climate models to simulate 1400 years of "natural" weather ... as soon as Osborn introduced global warming into the simulation, the index became strongly positive ... 'In effect, the only way you can get anything like the recent trend in the NAO is through global warming,' says Hurrell <of the National Center for Atmospheric Research in Boulder, Colorado>"
A report at the American Geophysical Union meeting in December 2001 confirmed that the trend was predicted by climate models - "It is consistent with most climate models' response to greenhouse gases", and suggested that increased warming will further enhance the effect.
|Are precipitation (rainfall) patterns changing?|
According the the US EPA's page on the subject, "During the 20th century, precipitation increased by about 0.5-1 percent per decade over most middle and high latitudes of the Northern Hemisphere's continents. Rainfall over the sub-tropical Northern hemisphere declined about 0.3 percent per decade, while no significant change occurred over the tropics". Despite this, percentage of Earth's land area stricken by serious drought more than doubled from the 1970s to the early 2000s (News Release). This occurred because increased temperatures increased the amount of evaporation.
Results from the climate model HADCM3 show that the greenhouse effect will likely cause an overall increase in precipitation, which will be greatest in high latitudes (towards the poles) The effect is likely to be lower in tropical and subtropical regions - with some of these regions actually experiencing decreased rainfall.
|Is wildlife being affected?|
A statistical analysis of wildlife behavioural patterns has shown that they are being affected by global warming. Over a period of around 45 years, the range of territory of 99 species of wildlife such as birds, butterflies and alpine herbs North America and Europe has shifted northward an average of 6.1 kilometres (3.79 miles) per decade, or to higher altitudes by an average of 6.1 meters (about 20 feet) per decade. The breeding dates (or floral blooming dates) of 172 species of migratory birds, amphibians and other animals studied showed an average shift of 2 days per decade earlier in the spring.
|Other Global Warming FAQ Topics|
|How does the current Climate Compare with that of the Past
How does the current temperature compare with the past 1000 years?
How has temperature and CO2 changed since the last ice age?
How does the current temperature compare with the past 400,000 years?
How does the current temperature compare with the past 600,000,000 years?
Have rapid increases in CO2 caused climate change in the past?
What is causing the increased warmth?
What are the predictions for the future?
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Last updated 10/03/06. By Tom Rees. Contact the author