The greenhouse effect

1. The greenhouse effect • To some extent, life on earth is contingent on the greenhouse effect – without it the earth’s surface temperature would be considerably below the freezing point of water. • Certain gases are opaque at infra-red light frequencies. These include methane, water vapour, nitrous oxides, ozone, and most importantly, CO2. Source: ARIC • Some short wave solar radiation is converted to long wave infra red at the earth’s surface. This is then absorbed by greenhouse gases, raising the atmospheric temperature. • As the proportion of greenhouse gases in the atmosphere increases, so does the potential for absorption of radiation, effectively raising the temperature. • This effect, and the dangers of increased carbon dioxide emissions wasoutlined by Svante Arrhenius over a century ago! (Arrhenius, 1896).

2. Climate change forcings • The effect of various factors on the climate are expressed as a forcing value (ie: to what extent they force global warming) • The forcing value is measured in Wm-2 (the increase in effective energy caused per square metre). • Forcings are discussed in more detail in the next chapter, on GCMs, but for now it is important to understand that there are many sources of climate forcing. • However, human activity has introduced significant radiative forcing to the atmosphere through various factors, and these will be touched upon in the next couple of slides. Source: Mauna Loa Observatory

3. Source: IPCC Third Assessment Report Greenhouse gases: methane • Since the beginning of the industrial revolution, and most particularly in the last century with the widespread use of the internal combustion engine, atmospheric concentrations of greenhouse gases have climbed massively. • Methane (CH4), is produced by domestic animals in large quantities. • Also produced by all anaerobic decomposition (notably in landfills, hydroelectric dams and rice paddies). (Prather et al.,1995). • Forcing increase relative to 1765 (Wm-2): • 19001960197019801990 • 0.10.240.300.360.42

4. Source: IPCC Third Assessment Report Greenhouse gases: nitrous oxide • Nitrous oxide (N2O) is naturally produced by biological functions in the soil and oceans. • Anthropogenic sources include industrial combustion, vehicle exhausts, biomass burning and the use of chemical fertilisers. • Forcing increase relative to 1765 (Wm-2): • 19001960197019801990 • 0.0270.0450.0540.0680.10

5. Carbon dioxide is emitted by all combustion, particularly fossil fuels used for engine fuels and energy generation. Burning of forests also emits CO2, as well as reducing their capacity for carbon sequestration. Greenhouse gases: carbon dioxide Source: IPCC Third Assessment Report • At this stage CO2 is the largest anthropogenic contribution to climatic forcing. Levels are higher than at any time in the past 450 000 years. • Forcing increase relative to 1765 (Wm-2): • 19001960197019801990 • 0.370.790.961.201.50

6. Greenhouse gases: others • Ozone plays an important role in reducing shortwave radiation influx by absorbing primarily ultraviolet light in the upper atmosphere (Chapman, 1930). However, as a pollutant in the lower atmosphere it also acts as a greenhouse gas. • The catalytic action of nitrous oxides, halocarbons and hydroxl ions (OH-) in the stratosphere catalytically destroys large quantities of ozone, thereby increasing solar radiation. • Halocarbons (CFCs and HCFCs), apart from destroying ozone in the upper atmosphere, are very strong greenhouse gases (thousands of times stronger than CO2) (IPCC, 1990). Furthermore, they are highly stable, taking decades to centuries to break down. • Forcing increase relative to 1765 (Wm-2) for CFC-11 and CFC-12 combined: 19001960197019801990 0.00.0120.0480.1110.202

7. Aerosols • Aerosols are small particles dispersed in the air (Kemp, 1994), and include dust, water, soot, sea crystals, and many others. • Aerosols typically generate a cooling effect on the atmosphere by either acting as seeds for cloud formation, or by directly reflecting solar radiation. • Although natural causes still generate significantly more aerosols than anthropogenic, there is some concern that the high levels of aerosol emission in the northern hemisphere (particularly where there is large amounts of burning such as the East, or dirty power generation) might be masking the actual effects of climate change. • This cooling effect is a very short term effect in comparison to other anthropogenic radiative forcings, and consequently climate change may paradoxically be accelerated to some extent by reduced emissions.

8. Sulphates and nitrates • Data from the Greenland ice sheet shows a dramatic increase in non-sea nitrate and sulphate concentrations since 1900 due to human activities. • These compounds are released through combustion of fossil fuels, and are primary components of acid rain • Ironically, these aerosols may be offsetting the global warming effects of CO2. Click to enlarge. Data from Mayewski et al, University of New Hampshire. See Mayewski et al (1986, 1990)

9. Thermal indicators: global air temperature • The mean global temperature has risen considerably in the past century (0.6°C ± 0.2°C) (IPCC, 2001). • The 1990s were the warm-est decade ever recorded . • Furthermore, proxy records indicate that temperatures have not reached current levels for at least the past thousand years. • In fact, proxy records indicate that the last time global temperature reached levels like the current conditions were at the peak of the last interglacial period, 124 000 years ago. • Note that proxy records for recent history corroborate instrumental temperature assessments. This indicates the accuracy of proxy interpretation methods.

10. Changes in precipitation • Globally, an increase in precipitation over land of approximately 2% has been observed over the last century (Jones and Hulme, 1996; Hulme et al., 1998). • However, this effect is localized, and such areas as the Sahel and sub-Saharan Africa have actually seen a decrease in rainfall. • Precipitation over the ocean can only properly be estimated from satellite observations, and as such, full records have only been available since 1987. • Current indications are that oceanic precipitation has increased as well, but it is too early to say with statistical certainty. • There may be a trend towards more intense rainfall events – whilst much of sub-Saharan Africa is getting less rain, it is also getting it in short bursts, rather than spread evenly over a season.

11. Interannual variability in the number of major hurricanes and the long-term average across the North Atlantic (Landsea et al., 1999). Climate change indicators: extreme weather • Tropical cyclones are associated with high winds and extreme rainfall, and have become high profile events of late. • However, an analysis of moderate and strong cyclones (≤ 980 hPa) shows no statistically significant trend. • A similar analysis of extra-tropical cyclones reveals a significant increase in the latter half of the 20th century. • It is unclear at this juncture whether this is part of a multi-decadal variation or a significant long-term trend.

12. Conclusions? • The international scientific community is now in firm agreement that global climate change is an established fact. • To some extent our understanding of future trends in the climate is dependent on the accuracy of the model we use to describe the processes, as will be described in the next chapter. • However, international political responses to this fact have been considerably less than spectacular at this juncture. • Given that climate change is inevitable, it is incumbent upon conservation planners to integrate the current extent of knowledge into their planning ventures wherever possible.