Recent climate change

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The three main indicators of global warming are temperature, precipitation, and sea level. One of the key aims of scientists over the last couple of decades has been to estimate how these have changed since the industrial revolution and to see if there is any evidence for global warming being to blame. Below is the evidence for each of these parameters.

Temperature

As we have seen, temperatures for the northern hemisphere have been reconstructed for the last thousand years, providing a context to the 20th century. Temperatures for the last 150 years have been estimated from a number of sources, both direct thermometer-based indicators and proxy-based. What is a proxy? As used here and elsewhere, it is short for proxy variable. The term 'proxy' is commonly used to describe a stand-in or substitute, as in 'proxy vote' or 'fighting by proxy'. In the same way, proxy variable in the parlance of climatology means a measurable 'descriptor' that stands in for a desired (but unobservable) 'variable', such as past ocean or land temperature. So there is an assumption that you can use the og proxy variable to estimate a climatic variable that you cannot | measure directly. So, as we will see below, you can use the thickness j| of tree rings as a way of estimating past land temperatures; in this 3 case, the tree-ring thickness is a proxy for temperature.

Thermometer-based indicators include sea-surface temperature (SST), marine air temperatures (MAT), land surface-air temperature, and temperatures in the free atmosphere, such as those measured by sensors on balloons. Borehole temperature measurements are defined as proxy-based because, despite the use of direct measurements of temperatures, these have been altered over time. Mathematical inversion procedures are required to translate the modern temperature in the boreholes into changes of ground temperature back through time. Other proxy-based methods include infrared satellite measurements and tree-ring width and thickness.

Thermometer-based measurements of air temperature have been recorded at a number of sites in North America and Europe as far back as 1760. The number of observation sites does not increase to sufficient worldwide geographical coverage to permit a global land average to be calculated until about the middle of the 19th century. SST and marine air temperatures were systematically recorded by ships from the mid-19th century, but even today the coverage of the southern hemisphere is extremely poor. All these data sets require various corrections to account for changing conditions and measurement techniques. For example, for land data each station has been examined to ensure that conditions have not varied through time as a result of changes in the measurement site, instruments used, instrument shelters, or the way monthly averages were computed, or the growth of cities around the sites, which leads to warmer temperatures caused by the urban heat island effect.

For SST and MAT there are a number of corrections that have to be applied. First, up to 1941 most SST temperature measurements f were made in sea water hoisted on deck in a bucket. Since 1941 I. most measurements have been made at the ships' engine water = intakes. Second, between 1856 and 1910 there was a shift from ? wooden to canvas buckets, which changes the amount of cooling m caused by evaporation that occurs as the water is being hoisted on sf deck. In addition, through this period there was a gradual shift from n sailing ships to steamships, which altered the height of the ship e decks and the speed of the ships, both of which can affect the evaporative cooling of the buckets. The other key correction that has to be made is for the global distribution of meteorological stations through time. As shown in Figure 14, the number of stations and their location varies greatly from 1870 to 1960. But by making these corrections it is possible to produce a continuous record of land-surface air and SST temperature for the last 130 years, which shows a global warming of 0.65°C ±0.05°C over this period.

What is so interesting about the 130-year temperature data set are the details, particularly as mentioned before the cooling observed in the 1960s and 1970s. One of the key tests for climate models, used to predict future climate changes, is whether they can reproduce the changes seen since 1870. These models are discussed in more detail

1870

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14. Global distribution of meteorological stations

1900

1900

in the next chapter but it should be noted that only by combining natural forcing (such as solar 11-year cycles and stratospheric aerosols from explosive volcanic eruptions), and anthropogenic forcing (greenhouse gases and sulphur aerosols) can the temperature record be simulated.

For the last 40 years balloon data has been available. In 1958 an initial network of 540 stations was set up to release rawindsondes, or balloons, which were regularly released to measure temperature, relative humidity, and pressure through the atmosphere to a height of about 20 km, where they burst. By the 1970s the network had grown to 700-800 stations reporting twice daily. The balloon data set shows a general surface and lower troposphere warming over the last 30 years of about 0.1-0.2 °C/10 years, while weak cooling is seen in the upper troposphere and strong cooling in the f stratosphere. I.

Satellite-based proxy records have been available for the last 20 ? years and have been the source of some key controversies in the m global warming debate. The advantage of satellite-mounted sf microwave sensors is that they have a global coverage, unlike the n balloons which are predominately land-based and in the northern e hemisphere. There are, however, some major problems with the microwave data set. First, the temperature record is based on eight different satellites, and despite overlapping measurement times, intercalibration between different instruments is problematic. Second, there is a spurious warming trend after 1990 of 0.03-0.04 °C which is due to a drift in the orbital times, and a spurious cooling trend of 0.12°C/decade due to the reduced altitude or height of the satellites caused by friction with the atmosphere. Third, the height within the atmosphere at which the microwave sensor measures temperature is affected by the amount of ice crystals and raindrops in the atmosphere. Hence, if the planet is warming up, moisture will be found at great altitude, and the microwave sensor would in fact measure temperature much higher in the atmosphere, i.e. in the colder parts of the troposphere, thus giving a smaller temperature increase than that which actually occurred. It is unsurprising that reports on satellite recorded global temperature trends for the last 30 years have changed, as every new paper published contains yet another correction that must be considered. For example, huge controversy occurred when Christy et al. (1995) deduced a global mean cooling trend of 0.05°C/decade for the period 1979-94, but obtained a warming trend of 0.09°C/decade over this period by removing the effects of El Nino and the climatic effect of the eruption of Mount Pinatubo. When the data set is corrected for decreasing satellite altitude, the global mean cooling turns into a warming of 0.07°C/decade. If the balloon, surface, and satellite data are compared, there is some agreement and it shows that the surface and lower troposphere have been warming up, while the stratosphere has been cooling down. An excellent summary of the corrections that have been made to each data set and why they were applied can be found in Harvey (2000).

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| The Intergovernmental Panel on Climate Change (IPCC) has j| collated all the published land-surface air and sea-surface 3 temperatures from 1861 to 1998, with all the corrections discussed above. This data is shown relative to the average temperature between 1961 and 1990 in Figure 13, and, as you can see, there has been a sharp warming from the start of the 1980s onwards. The mean global surface temperature has increased by about 0.3 to 0.6°C since the late 19th century. Including the evidence from balloons and satellites, there seems to be a 0.2 to 0.3°C increase over the last 40 years, which is the period with most reliable data. Recent years have been among the warmest since 1860 - the period for which instrumental records are available. This warming is evident in both sea-surface and land-based surface air temperatures. Indirect indicators, such as borehole temperatures and glacier shrinkage, provide independent support for the observed warming. It should also be noted that the warming has not been globally uniform. The recent warming has been greatest between 40°N and 70°N latitude, though some areas such as the North Atlantic Ocean have cooled in the recent decades.

Precipitation

There are two global precipitation data sets: 'Hulme' and the

'Global Historical Climate Network' (GHCN). Unfortunately, unlike temperature, rainfall and snow records are not as well documented and the records have not been carried out for as long. It is also known that precipitation over land tends to be underestimated by up to 10-15% owing to the effects of airflow around the collecting dish. The gradual realization and correction of this effect has produced a spurious upward trend in global precipitation. After correction, there is an overall increase of precipitation of 1% over land, which is so small that it cannot be distinguished from zero, i.e. no change. A detailed view suggests that, taking an average over the Earth's land surface, precipitation increased from the start of the century up to about 1960, but has decreased since about 1980.

But yet again, as with main key data sets concerning global e warming, we have a huge gap, which is due to the lack of data on ee precipitation over the oceans. However, what is observed are some ce significant changes in where the precipitation has occurred (Figure 3

15). It seems that precipitation has increased over land at high f latitudes in the northern hemisphere, especially during the cold e c season. One study also suggested that there was an increase in n g the amount of rain falling during heavy rain events, especially *

in the USA, the former Soviet Union, and China. Decreases in precipitation occurred after the 1960s over the subtropics and the tropics from Africa to Indonesia. These changes are consistent with available data analyses of changes in stream flow, lake levels, and soil surface. In terms of snowfall, Antarctic is a big winner with an increase of 5-20% over the last two decades, while Greenland has lost about 20% of its snow accumulation over the last 50 years.

Relative sea level

The IPCC has also put together a key data set of sea level. In general it shows that over the last 100 years, the global sea level has risen by about 4 to 14 cm (Figure 16). But sea-level change is difficult to measure, as relative sea-level changes have been derived mainly from tide-gauge data. In the conventional tide-gauge system, the

15. Changes in precipitation over land a) 1955-1974 to 1975-1994 and b) 1900 to 1994

sea level is measured relative to a land-based tide-gauge benchmark. The major problem is that the land surface is much more dynamic that one would expect, with a lot of vertical movements, and these get incorporated into the measurements. Vertical movements can occur as a result of normal geological compaction of delta sediments, the withdrawal of groundwater from coastal aquifers (both of which are discussed in more detail in Chapter 6, Coastline section), uplift associated with colliding tectonic plates (the most extreme of which is mountain building such as the Himalayas), or ongoing postglacial rebound and compensation elsewhere associated with the end of the last ice age. The latter is caused by the rapid removal of weight when the giant ice sheets melted, so that the land which has been weighed down slowly rebounds back to its original position. An example of this is Scotland, which is rising at a rate of 3 mm/year while England is still sinking at a rate of 2 mm/year, despite the Scottish ice sheet having melted 10,000 years ago. Again, using a number of corrections, the global tide-gauge network suggests that the rise in sea level since the beginning of the 20th century could be as much as 18 cm (~1.8±0.1 mm/year). On this timescale, the warming and the consequent thermal expansion of the oceans may account for about 2-7 cm of the observed sea-level rise, while the observed retreat of glaciers may account for about 2-5 cm. Other factors are more difficult to quantify. The rate of observed sea-level rise suggests that there may have been a net positive contribution from the huge ice sheets of Greenland and Antarctica, but observations of these ice sheets suggest that there may have been a net expansion e which would have contributed -0.05 mm/year to global sea level I. over the last 100 years. The ice sheets remain a major source of = uncertainty in accounting for past changes in sea level because of ? insufficient data about these ice sheets over the last 100 years. m u

One of the biggest unknowns of global warming is whether the n massive ice sheets over Greenland and Antarctica will melt. A key e indicator of the expansion or contraction of these ice sheets is the sea ice that surrounds them. The state of the cryosphere (or the global ice) is extremely important, as shrinking of ice on land causes the sea level to rise. Unfortunately, submarines have already recorded a worrying thinning of the polar ice caps. Sea-ice draft is the thickness of the part of the ice that is submerged under the sea. Therefore, in order to understand the effects of global warming on the cryosphere it is important to measure how much ice is melting in the polar regions. Comparison of sea-ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates 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 portions of the Arctic Ocean, from 3.1 m in 1958-76 to 1.8 m in the 1990s. In summary, ice draft in the 1990s is over a

Global Warming 1910

1910 1930 1951 Yea

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16. Estimated sea-level rise 1910-1990

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metre thinner than two to four decades earlier. The main draft has decreased from over 3 metres to less than 2 metres and the volume is down by some 40%. In addition, in 2000, for the first time in recorded history, a hole large enough to be seen from space opened in the sea ice above the North Pole. Unfortunately, because satellite records are so short, we do not know if this is a frequent natural occurrence or indicative of significant melting of Arctic sea ice. Moreover, measurements of the size of Greenland suggest that it is shrinking, particularly at its coastal margins.

Other evidence for global warming

Other evidence for global warming comes from permafrost regions and weather patterns such as particular storm records. In the high latitude and high altitude areas permafrost exists, where it is so cold f that the ground is frozen solid to a great depth. During the summer I. months only the top metre or so of the permafrost gets warm =

enough to melt, and this is called the active layer. Already in Alaska ? there seems to have been a 3°C warming down to at least a metre m over the last 50 years, showing that the active layer has become sf deeper. With the massive increases in atmospheric CO2 predicted n for the future, it is likely that there will be increases in the thickness e of the active layer of the permafrost or perhaps, in some areas, the complete disappearance of so-called discontinuous permafrost over the next century. This widespread loss of permafrost will produce a huge range of problems in local areas, as it will trigger erosion or subsidence, change hydrologic processes, and release into the atmosphere even more CO2 and methane trapped as organic matter in the frozen layers. Hence changes in permafrost will reduce the stability of slopes and thus increase incidence of slides and avalanches. A more dynamic cryosphere will increase the natural hazards for people, structures, and communication links. Already buildings, roads, pipelines, such as the oil pipelines in Alaska, and communication links are being threatened.

There is evidence too that our weather patterns are changing. For example, in recent years massive storms and subsequent floods have hit China, Italy, England, Korea, Bangladesh, Venezuela, and Mozambique. In England in 2000, floods classified as 'once in 30-year events' occurred twice in the same month. Moreover, the winter of 2000/1 was the wettest six months recorded in Britain since records began in the 18th century, while in the summer of 2003 Britain recorded the first ever temperature of 100°F since records began. In addition, on average, British birds nest 12±4 days earlier than 30 years ago. Insect species - including bees and termites - that need warm weather to survive are moving northward, and some have already reached England by crossing the Channel from France. Glaciers in Europe are in retreat, particularly in the Alps and Iceland. Ice cover records from the Tornio River in Finland, which has been recorded since 1693, show that the spring thaw of the frozen river now occurs a month earlier.

There is also evidence that more storms are occurring in the | northern hemisphere. Wave height in the North Atlantic Ocean has j| been monitored since the early 1950s, from lightships, Ocean 3 Weather Stations, and more recently satellites. Between the 1950s and 1990s the average wave height increased from 2.5 to 3.5 m, an increase of 40%. Storm intensity is the major control over wave height, which provides evidence for an increase in storm activity over the last 40 years. This is supported by German scientists who suggested that storm-generated ocean waves pounding the coasts of Europe produce long-wave vibrations which are picked up by the sensitive equipment set up to record earthquakes. From this evidence they calculated the number of storm days per month during the winter. It seems that over the last 50 years these have increased from seven to 14 days per month. This also fits with the observed increase in winter extratropical cyclones, i.e. those occurring in the mid-latitudes, which have increased markedly over the last hundred years, with significant increases in both the Pacific and Atlantic sectors since the early 1970s. There has, however, in contrast, been a slight downturn in the number of hurricanes over the last 50 years.

17. Mozambique floods of 2000

What do the sceptics say?

One of the best ways to summarize the evidence for global warming and to persuade you, the reader, that there is evidence that humanity has already altered global climate, is to review what the sceptics say against the global warming hypothesis:

1. Ice-core data suggest atmospheric CO2 responds to global temperature, therefore, atmospheric CO2 cannot cause global temperature changes.

last glacial period shows that the major stepwise increases occur at the same time as warming in Antarctica. It is known that during the last de-glaciation, gradual warming in Antarctica occurred before step-like warming in the northern hemisphere (Figure 18). There is, therefore, excellent evidence that atmospheric carbon dioxide increases before overall global temperatures rise and the ice sheets begin to melt. In fact, there is clear evidence that Antarctic temperatures and atmospheric carbon dioxide levels are in step (Figure 18), demonstrating the central role of carbon dioxide as a climate amplifier. Moreover, time-series analysis of the last four glacial-interglacial cycles by Professor Shackleton at Cambridge University suggests atmospheric carbon dioxide response up to 5,000 years before variations in global ice sheets. This has prompted many palaeoclimatologists to re-evaluate the role of atmospheric carbon dioxide, placing it now as a primary driving force of past climate instead of a secondary response and feedback.

2. Every data set showing global warming has been corrected or tweaked to achieve this desired result.

A detailed examination of the ice-core CO2 data at the end of the

For people who are not regularly involved in science this seems to be the biggest problem with the whole 'global warming has happened' argument. As I have shown, all the data sets covering the last 150 years require some sort of adjustment. This, though, is part of the

18. Ice core records showing CO2 in phase with Antarctic warming. A) 818O and 8D = temperature records, ACO2 changing atmospheric carbon dioxide levels, higher curve taking into account coral reef and land vegetation changes since the last ice age. B) rate of change of carbon dioxide most of which occurs in three large pulses scientific process. For example, if great care had not been taken over the spurious trends in the global precipitation data base we would now assume that global precipitation was increasing. Moreover, as science moves forward incrementally, it gains more and more understanding and insight into the data sets it is constructing. This constant questioning of all data and interpretations is the core strength of science: each new correction or adjustment is due to a greater understanding of the data and the climate system and thus each new study adds to the confidence that we have in the results. This is why the IPCC report refers to the 'weight of the evidence', as our confidence in science increases if similar results are obtained from very different sources.

3. Solar output and sunspot activity control the past temperatures. This is something both the sceptics and non-sceptics agree on. Of og course sunspots and also volcanic activity influence past | temperatures. For example, the cooling of the 1960s and 1970s is j| clearly linked to changes in the sunspot cycle. The difference 3 between the two camps is that the sceptics put more weight on the importance of these natural variations. Though great care has been taken to understand how the minor variations in solar output affect global climate, this is still one of the areas which contain many unknowns and uncertainties. However, climate models combining our current state-of-the-art knowledge concerning all radiative forcing, including greenhouse gases (see Table 1 on pages 16 and 17) and sunspots, are able to simulate the global temperature curve for the last 130 years. Figure 19 shows the separate natural and anthropogenic forcing on global climate for the last 130 years and the combination of the two. This provides confidence in both models and also an understanding of the relative influence of natural versus anthropogenic forcing.

4. Satellite data casts doubt on the models.

Again, before the satellite data was clearly understood it did suggest

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19. Simulated annual global mean surface temperatures compared to observed temperatures

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*" Virtually certain (probability > 99%) Very likely (probability > 90% but < 99%) Likely (probability > 6«% but < 90%) Medium likelihood (probability > 33% but <66%)

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J ? 2% increase in total cloud amount over Ihe ocean »nee 1952

1 upper troposphere: "no s>gmlicant global trends since i960;

15% increase in tropics (10*N to 10*S) i troposphere: 4many redora with increases since aboul 1960

2% Increase In lotal cloud amount . over land during Ihe 20th century

J ? 2% increase in total cloud amount over Ihe ocean »nee 1952

1 no systematic la rge-scale change «n tornadoes. Ihunder-days, hail near-surface

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** no widespread Changes HOfMI storm frequency t during the 20th eemury

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N. Hemisphere, 1975 to 1995

'** virtually certain (probability > 99%) *' Very likely (probability > 90% but < 99%) * Likely (probability > 66% but < 90%) ? Medium likelihood (probability > 33% but < 66%)

20. Schematic of observed variations of the temperature indicators and hydrological and storm-related indicators that over the last 20 years there had been a slight cooling. The iterative process of science, i.e. the re-examination of data and the assumption concerning the data, clearly showed that there were some major inconsistencies within the satellite data; first, as a result of trying to compare the data from different instruments on different satellites and, second, because of the need to adjust the altitude of the satellite as its orbit shrinks as a result of friction with the atmosphere. The final problem with the satellite data is that 20 years is just too short a time period to find a temperature trend with any confidence. This is because climatic cycles or events will have a major influence on the record and will not be averaged out; for example, the sunspot cycle is 11 years, El Nino-Southern Oscillation is 3-7 years, and the North Atlantic Oscillation is ten years. So which of these cycles is picked up by the 20-year satellite data will strongly influence the direction of the temperature trend. f

Figure 20 summarizes the current state of knowledge concerning = the climatic changes that have occurred over the last 100 years ? both in temperatures and the hydrological cycle, while Figure 21 m shows the geographic locations where evidence of global warming jf over the last 100 years has been found. One of the key arguments for n me that significant warming and other climatic changes have e occurred over the last 100 years is the weight of evidence from so many diverse data sets. When the last 100 years are compared with the last 1,000 years it is very clear that something completely different is occurring. The evidence suggests that natural climate forcings such as sunspots and volcanic eruptions have been similar for the last millennium. This leaves only one alternative - that greenhouses gases, with their known radiative forcing, have already influenced global climate. From the huge amount of published scientific evidence the IPCC (2001) has concluded: 'In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely [6090% confidence] to be due to the increase in greenhouse gas concentration.'

Hydrology Sea ice Animals Plants Studies covering Studies using and glaciers large areas remote sensing

21. Locations at which systematic long-term studies meet stringent criteria documenting recent temperature-related regional climate change impacts on physical and biological systems

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