global 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.
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".
Is the Arctic warming?
The 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 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.
Is Arctic ice melting?
Currently, satellite data show that Arctic ice is decreasing at the rate 9% per decade. 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?
"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...
Are the Oceans warming?
On the whole, they show a similar trend to that of global land surface temperatures. The oceans are also warming at depth....
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.
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...
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".
Is the North Atlantic Oscillation behaving normally?
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...
"In recent years ... the NAO has been in a period of unprecedented activity. Its flips are bigger...
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?
...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.
Is wildlife being affected?
A statistical analysis of wildlife behavioural patterns has shown that they are being affected by global warming.
How does the current temperature compare with the past 1000 years?
...all these reconstructions agree that the global mean temperatures have declined gradually over the past 1000 years, before increasing rapidly more recently. In the northern hemisphere, there seems to have been a more pronounced warm period early in the millennium and cool period in the latter part of the millennium (the 'Medieval Warm Period' and 'Little Ice Age', respectively). These do not appear to have been global events (see this review), and instead are likely to reflect changes in the North Atlantic Oscillation and linked to changes in solar activity
Before the onset of the industrial revolution, and the resultant increase in greenhouse gases, global temperature changes are thought to be largely a result of changes in solar activity, volcanic activity, and deforestation (which caused a gradual cooling, especially in the last few hundred years), overlaid onto a background of internal climate 'noise'.
How has temperature and CO2 changed since the last ice age?
For the past ten thousand years, however, the earth's temperature and atmospheric CO2 has been relatively stable – although temperatures have varied over a range of 4ºC (although some of this variation is probably an artefact due to inevitable measurement errors). The causes of these fluctuations are not known with certainty, but are likely to be due to combinations of variation in solar activity and periodic changes in ocean currents.
The near-vertical red line at the far left marks the rise in atmospheric CO2 since the start of the industrial revolution.
Has a sudden rise in greenhouse gases ever caused climate change? TOP
The complicated nature of past climate has lead some people to suggest that changing levels of greenhouse gases have only a small effect on temperature, especially when compared with other causes of temperature change. To test whether this is so, it is important to know whether a rapid increase in greenhouse gases, such as is now occurring, can actually cause significant climate change. Two events in the past suggest that this is, indeed, the case.
....
Late Palaeocene Thermal Maximum (LPTM, 55 million years ago)
Scientists have long known that there was a short, but dramatic period of warming at the end of the Palaeocene era. This is now believed to have resulted from several massive releases of methane from the sea floor, probably as a result of continental drift, but possibly as a result of volcanic activity. For more information, see Earth 's ancient heat wave gives a taste of things to come, this NASA story and the paper by Bains et al, Science 1999. The changes in sea surface temperatures fit well with predictions from climate models, according to research published in 2003. Although there are several similarities to today's warming, one major difference is the timescale: the release of methane took place over a period of several thousand years. There was no mass extinction, probably because plants had sufficient time to 'migrate' northwards to more hospitable climates.
Is there a natural greenhouse effect?
Yes ...Most climate scientists think that, over the next century, mankind's emissions of greenhouse gases will increase the greenhouse effect by 5-15%, causing a warming of 1.8-5.4ºC.
Is water vapour the most important greenhouse gas? TOP
The degree to which water vapour will amplify the effects of increases in other greenhouse gases is a crucial issue, but one that is currently far from resolved. Most climate models show that doubling the concentration of CO2 in the atmosphere will increase temperatures by 1.1ºC, and that water vapour feedback will increase this to 1.8ºC (further amplification comes as snow and ice is melted, thereby reducing albedo).
Are greenhouse gases increasing? TOP
At the end of the last ice age, the concentration of CO2 increased by around 100 ppm (parts per million) over around 8,000 years, or around 1.25 ppm per century. Since the start of the industrial revolution, the rate of increase has accelerated markedly: since 1860, the concentration of CO2 has increased by around 80 ppm, just over 50 ppm per century. The rate of CO2 accumulation has continued to increase, and it currently stands at around 150 ppm/century – over 200 times faster than the background rate for the past 15,000 years.
What is causing the increase in CO2 and other greenhouse gases?
.... This strongly suggests that the cause is different to that which has caused greenhouse gases to increase in the past.
...Major sources of methane include rice paddies, livestock, and landfill sites (see methane cycle and Robert Grumbine's methane FAQ).
Is the Earth absorbing more radiation than it emits?
The analysis of the spectrum of radiation emitted from the earth shows that those frequencies which are predicted to be absorbed by greenhouse gases really are being absorbed.
Is the recent warming caused by changes in solar radiation?
It is clear that there has been no recent rise in solar activity that could explain the recent rise in temperature.
...
These reconstructions of solar activity have allowed scientists to investigate the relative impact of solar activity and greenhouse gases on current climate change. Several studies have investigated the relationship between changes in solar activity, volcanoes and temperatures over the past 1000 years and have concluded that, although changes in solar activity can explain much of the temperature changes, there is a 'residual' unexplained temperature increase in the 20th century.
...
"While the natural (solar and volcanic) forcings appear to be important factors governing the natural variations of temperatures in past centuries, only human greenhouse gas forcing alone ... can statistically explain the unusual warmth of the past few decades". A similar analysis by Crowley et al concludes that there is "a very large late 20th century warming that closely agrees with the response predicted from greenhouse gas forcing.”
.... Similarly, two recent studies of ocean temperatures have found that the observed increase is best explained by the effect of greenhouse gases.
Are climate models accurate? TOP
... even by 1998 they were able to simulate the main features of our climate, including, according to a pamphlet from the World Meteorological Association, 'the seasonal cycle of temperature, the formation and decay of the major monsoons, the seasonal shift of the major rain belts and storm tracks, the average daily temperature cycle, and the variations in outgoing radiation at high elevations in the atmosphere as measured by satellites.
.....
Nevertheless, there is still a great deal of uncertainty in predicting future climate.
Although there is plenty of room for error, it is important to realise that the effect of any errors is unknown. That is, it could be that warming is less severe than predicted. However, and just as likely, the warming could be more severe than predicted.
Will increased plant growth absorb the excess CO2?
... the natural world is already absorbing more than it emits, and predictions of climate change are based on the assumption that this effect will continue. Unfortunately, there are several problems.
Firstly, it's not enough. Data from Mauna Loa show that carbon dioxide levels have steadily increased throughout the century. Studies have shown that the ability of trees to soak up excess carbon is limited by the availability of other nutrients, as discussed in this article from National Geographic.
Secondly, there's more to greenhouse gas than CO2, as this link shows. Methane, a very important greenhouse gas, has doubled in concentration since the start of the industrial age.
Thirdly, we are actively reducing biomass by cutting down forests and draining marshland – deforestation in the 1980s was responsible for around 1.6 to 1.8 GtC emissions, according a report from the Edinburgh Centre for Carbon Management. In the 1990s, changing land use meant that deforestation has been balanced by reforestation of disused farmland, and there has been a net absorption of 1.4 billion tonnes. However, as these new forests mature, it is predicted that the balance will shift once again to a net loss of CO2 to the atmosphere.
Fourthly, although forests contain more CO2 than grassland, they are darker. This means that they absorb more heat. Scientists from the USA's Lawrence Livermore Laboratories report that deforestation likely played an important part in the global cooling that took part over the last millennium.
Finally, and most importantly, climate change is likely to have important effects on the carbon cycle. For example, as the oceans warm they will release CO2, and as the permafrost thaws it will start to decay, releasing large amounts of CO2. Peat marshes, too, are likely to release increasing amounts of CO2 as they warm (see this news report). Although the exact effects of these changes are hard to calculate, the current estimate from the HADCM3 model is that the net effect will be to increase global by a further 3ºC over the next 100 years – making the total increase up to 8ºC.
All the world's meteorological and climatic centres agree that global surface temperatures have risen by about 0.6ºC over the past 100 years.
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".
Is the Arctic warming?
The 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 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.
Is Arctic ice melting?
Currently, satellite data show that Arctic ice is decreasing at the rate 9% per decade. 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?
"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...
Are the Oceans warming?
On the whole, they show a similar trend to that of global land surface temperatures. The oceans are also warming at depth....
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.
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...
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".
Is the North Atlantic Oscillation behaving normally?
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...
"In recent years ... the NAO has been in a period of unprecedented activity. Its flips are bigger...
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?
...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.
Is wildlife being affected?
A statistical analysis of wildlife behavioural patterns has shown that they are being affected by global warming.
How does the current temperature compare with the past 1000 years?
...all these reconstructions agree that the global mean temperatures have declined gradually over the past 1000 years, before increasing rapidly more recently. In the northern hemisphere, there seems to have been a more pronounced warm period early in the millennium and cool period in the latter part of the millennium (the 'Medieval Warm Period' and 'Little Ice Age', respectively). These do not appear to have been global events (see this review), and instead are likely to reflect changes in the North Atlantic Oscillation and linked to changes in solar activity
Before the onset of the industrial revolution, and the resultant increase in greenhouse gases, global temperature changes are thought to be largely a result of changes in solar activity, volcanic activity, and deforestation (which caused a gradual cooling, especially in the last few hundred years), overlaid onto a background of internal climate 'noise'.
How has temperature and CO2 changed since the last ice age?
For the past ten thousand years, however, the earth's temperature and atmospheric CO2 has been relatively stable – although temperatures have varied over a range of 4ºC (although some of this variation is probably an artefact due to inevitable measurement errors). The causes of these fluctuations are not known with certainty, but are likely to be due to combinations of variation in solar activity and periodic changes in ocean currents.
The near-vertical red line at the far left marks the rise in atmospheric CO2 since the start of the industrial revolution.
Has a sudden rise in greenhouse gases ever caused climate change? TOP
The complicated nature of past climate has lead some people to suggest that changing levels of greenhouse gases have only a small effect on temperature, especially when compared with other causes of temperature change. To test whether this is so, it is important to know whether a rapid increase in greenhouse gases, such as is now occurring, can actually cause significant climate change. Two events in the past suggest that this is, indeed, the case.
....
Late Palaeocene Thermal Maximum (LPTM, 55 million years ago)
Scientists have long known that there was a short, but dramatic period of warming at the end of the Palaeocene era. This is now believed to have resulted from several massive releases of methane from the sea floor, probably as a result of continental drift, but possibly as a result of volcanic activity. For more information, see Earth 's ancient heat wave gives a taste of things to come, this NASA story and the paper by Bains et al, Science 1999. The changes in sea surface temperatures fit well with predictions from climate models, according to research published in 2003. Although there are several similarities to today's warming, one major difference is the timescale: the release of methane took place over a period of several thousand years. There was no mass extinction, probably because plants had sufficient time to 'migrate' northwards to more hospitable climates.
Is there a natural greenhouse effect?
Yes ...Most climate scientists think that, over the next century, mankind's emissions of greenhouse gases will increase the greenhouse effect by 5-15%, causing a warming of 1.8-5.4ºC.
Is water vapour the most important greenhouse gas? TOP
The degree to which water vapour will amplify the effects of increases in other greenhouse gases is a crucial issue, but one that is currently far from resolved. Most climate models show that doubling the concentration of CO2 in the atmosphere will increase temperatures by 1.1ºC, and that water vapour feedback will increase this to 1.8ºC (further amplification comes as snow and ice is melted, thereby reducing albedo).
Are greenhouse gases increasing? TOP
At the end of the last ice age, the concentration of CO2 increased by around 100 ppm (parts per million) over around 8,000 years, or around 1.25 ppm per century. Since the start of the industrial revolution, the rate of increase has accelerated markedly: since 1860, the concentration of CO2 has increased by around 80 ppm, just over 50 ppm per century. The rate of CO2 accumulation has continued to increase, and it currently stands at around 150 ppm/century – over 200 times faster than the background rate for the past 15,000 years.
What is causing the increase in CO2 and other greenhouse gases?
.... This strongly suggests that the cause is different to that which has caused greenhouse gases to increase in the past.
...Major sources of methane include rice paddies, livestock, and landfill sites (see methane cycle and Robert Grumbine's methane FAQ).
Is the Earth absorbing more radiation than it emits?
The analysis of the spectrum of radiation emitted from the earth shows that those frequencies which are predicted to be absorbed by greenhouse gases really are being absorbed.
Is the recent warming caused by changes in solar radiation?
It is clear that there has been no recent rise in solar activity that could explain the recent rise in temperature.
...
These reconstructions of solar activity have allowed scientists to investigate the relative impact of solar activity and greenhouse gases on current climate change. Several studies have investigated the relationship between changes in solar activity, volcanoes and temperatures over the past 1000 years and have concluded that, although changes in solar activity can explain much of the temperature changes, there is a 'residual' unexplained temperature increase in the 20th century.
...
"While the natural (solar and volcanic) forcings appear to be important factors governing the natural variations of temperatures in past centuries, only human greenhouse gas forcing alone ... can statistically explain the unusual warmth of the past few decades". A similar analysis by Crowley et al concludes that there is "a very large late 20th century warming that closely agrees with the response predicted from greenhouse gas forcing.”
.... Similarly, two recent studies of ocean temperatures have found that the observed increase is best explained by the effect of greenhouse gases.
Are climate models accurate? TOP
... even by 1998 they were able to simulate the main features of our climate, including, according to a pamphlet from the World Meteorological Association, 'the seasonal cycle of temperature, the formation and decay of the major monsoons, the seasonal shift of the major rain belts and storm tracks, the average daily temperature cycle, and the variations in outgoing radiation at high elevations in the atmosphere as measured by satellites.
.....
Nevertheless, there is still a great deal of uncertainty in predicting future climate.
Although there is plenty of room for error, it is important to realise that the effect of any errors is unknown. That is, it could be that warming is less severe than predicted. However, and just as likely, the warming could be more severe than predicted.
Will increased plant growth absorb the excess CO2?
... the natural world is already absorbing more than it emits, and predictions of climate change are based on the assumption that this effect will continue. Unfortunately, there are several problems.
Firstly, it's not enough. Data from Mauna Loa show that carbon dioxide levels have steadily increased throughout the century. Studies have shown that the ability of trees to soak up excess carbon is limited by the availability of other nutrients, as discussed in this article from National Geographic.
Secondly, there's more to greenhouse gas than CO2, as this link shows. Methane, a very important greenhouse gas, has doubled in concentration since the start of the industrial age.
Thirdly, we are actively reducing biomass by cutting down forests and draining marshland – deforestation in the 1980s was responsible for around 1.6 to 1.8 GtC emissions, according a report from the Edinburgh Centre for Carbon Management. In the 1990s, changing land use meant that deforestation has been balanced by reforestation of disused farmland, and there has been a net absorption of 1.4 billion tonnes. However, as these new forests mature, it is predicted that the balance will shift once again to a net loss of CO2 to the atmosphere.
Fourthly, although forests contain more CO2 than grassland, they are darker. This means that they absorb more heat. Scientists from the USA's Lawrence Livermore Laboratories report that deforestation likely played an important part in the global cooling that took part over the last millennium.
Finally, and most importantly, climate change is likely to have important effects on the carbon cycle. For example, as the oceans warm they will release CO2, and as the permafrost thaws it will start to decay, releasing large amounts of CO2. Peat marshes, too, are likely to release increasing amounts of CO2 as they warm (see this news report). Although the exact effects of these changes are hard to calculate, the current estimate from the HADCM3 model is that the net effect will be to increase global by a further 3ºC over the next 100 years – making the total increase up to 8ºC.
posted by david at 9:51 AM
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