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The statements set out below are effectively certain. Ask anyone who denies that global warming is real, or rejects the evidence that it is caused by human activities,  to provide a reference to a refereed scientific article that contradicts any of these statements.

To find out about the changes we are already seeing in our climate, take a look at the 2009 State of the Climate report published by NOAA (National Oceanic and Atmospheric Administration) in the June 2010 issue of the Bulletin of the American Meteorological Society. This annual report provides a broad look at the Earth's climate from the top of the atmosphere to the depths of the ocean, providing insight and perspective that helps us better understand what is happening to our climate. 

1. Carbon dioxide (CO2) is a radiatively active gas that absorbs infra-red radiation i.e. it is a greenhouse gas.

In 1859, careful laboratory work by John Tyndall in the UK identified several gases that could trap heat, coming from the Earth’s surface, in the atmosphere. One of these was CO2, although its concentration in the atmosphere is less than four parts in ten thousand. Just as a sheet of paper will block more light than an entire pool of clear water, so the CO2 significantly reduces the amount of heat that escapes from the atmosphere into space.

Figure 1 - A simplified model of the greenhouse gas effect (See Note [1]) 

2. Atmospheric CO2 concentrations are increasing, and have done so since the beginning of the industrial revolution.

Measurements on bubbles of air trapped in Antarctic ice cores show that before the beginning of the industrial revolution (mid-18th century) the atmospheric CO2 concentration had not exceeded 280 parts per million (ppm) for hundreds of thousands of years. However, it began to increase steadily from the early 19th century. In the late 1950s Charles David Keeling developed techniques and set up a system to regularly measure atmospheric CO2 concentrations in Hawaii and elsewhere. This series of measurements has been continued to the present day. The measurements show that since 1959 CO2 has increased from 313 to 387 ppm, almost 40 per cent more than the level in pre-industrial times, and continues to increase.

Figure 2 - Graph showing how atmospheric CO2 has increased rapidly since the 18th century and is greater today than at any time in the last 650,000 years (as indicated by the grey bar) (See Note [2])

 

3. The combustion of fossil fuels is today the largest single cause of emissions of carbon dioxide (CO2).

This statement is based on the observation that vegetation preferentially takes up from its surroundings more of one form of carbon (the isotope 12C) than another (13C). Hence the CO2 produced by burning fossil fuels, such as oil and gas which are derived indirectly from plant material, contains more 12C than 13C than the CO2 derived from most other carbon sources (emissions from the oceans, weathering of carbonate rocks, gases released by volcanoes). Thus measurements of changes in the ratio of 13C:12C in the atmosphere can indicate the origin of the increasing atmospheric CO2 concentrations. From 1978 to 2003 the 13C:12C ratio has steadily decreased around the world. The ratio first started to drop to values not seen for at least 500 years in the mid-19th century. This is a good indication that fossil fuel burning is influencing atmospheric concentrations of CO2. Other measurements and arguments based on carbon isotopes also suggest the increase in atmospheric CO2 is not derived from decaying vegetation or CO2 escaping from the oceans.

Figure 3 - Graph showing how oceanic delta 13C (blue, red and green lines and symbols) has decreased since 1850. delta13C is a measure of the 13C:12C ratio. (See Note [3]) 

4. Human-caused CO2 emissions are sufficient to account for the observed change in atmospheric CO2.

Globally it is estimated that human activities currently result in emissions of around 31 billion tonnes (31 Gt) of CO2 every year whereas the atmosphere accumulates around 16.5 Gt of this amount. The remainder is taken up by natural sinks such as the oceans (11 Gt) and vegetation and soils (3.5 Gt). Thus it is clear that human activities emit more than enough carbon dioxide to explain the increase in atmospheric CO2.

Figure 4 - The top graph shows the growth of atmospheric carbon (CO2) since 1960. The bottom graph shows carbon emissions from burning fossil fuels, cement manufacture and agriculture since 1960. The sum of the latter far exceeds the former because natural sinks absorb the difference. (See Note [4]) 

5. Increases in absorption of infra-red radiation in the atmosphere contribute to global warming (i.e. they increase the greenhouse effect).

Satellite measurements show that as the concentrations of greenhouse gases, particularly CO2, increase in the atmosphere, more infra-red (heat) radiation from the Earth’s surface is absorbed, instead of being radiated into space. This heats up the atmosphere.

Figure 5 - Representation of the decrease in infrared radiation (heat) leaving the Earth between 1970 and 1997 for different greenhouse gases. (See Note [5]) 

6. Global warming is occurring at rates that are unprecedented in human history (and, as far as we can tell, in earlier times).

Since the mid-19th century, plots of global average surface temperature against time have followed a curve that shows that the rate of temperature increase has increased over time. It is also certain that since the mid-19th century atmospheric CO2, which can be considered as an indirect measure of global average surface temperature, has increased faster than at any time in the last 10,000 years. It is difficult to be completely sure that current rates of increase in temperature and CO2 concentrations are faster than at any time in the geological past, partly because of problems with dating geological samples accurately enough. However, one estimate suggests that today atmospheric CO2 is increasing 20,000 times faster than it did from natural causes in the last 65 million years.

Figure 6 - Graphs showing how globally the oceans and the land have warmed since 1850. The two panels are based on five and four independent analyses of available data, respectively. (See Note [6]) 

7. The amount of warming observed is quantitatively consistent with the observed changes in greenhouse gas concentrations (and well founded estimates of the greenhouse effect).

Quantitative modelling of the Earth’s climate system matches the changes in global average surface temperature observed over at least the last quarter of the 20th century only if humankind’s contribution to atmospheric greenhouse gas concentrations is factored into the calculations. This contribution is in addition to all known natural sources of climate variation such as volcanic eruptions and variability of solar output. The latter sources have a smaller and less lasting effect on the climate than man-made greenhouse gases.

Figure 7 - Comparisons of observed 20th century global temperatures (black lines) with modelled global temperatures for the whole Earth, the land and the oceans (red and blue bands). Only when mankind’s emissions are included do the models fit the observations. (See Note [7]) 

The above list is based on texts produced by:

  1. Professor John Shepherd FRS OBE, a Deputy Director of the Tyndall Centre for Climate Change Research and a former Director of the National Oceanography Centre Southampton,
  2. UK Meteorological Office (http://www.metoffice.gov.uk/research/climate)
  3. IPCC (Summary for Policymakers and Technical Summary: WG 1. Climate Change 2007: The Physical Science Base: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK and New York, NY, USA, Cambridge University Press, page 81). 

Notes to Figures: more detailed explanations and sources

[1] From Frequently Asked Question 1.3, What is the greenhouse effect?, IPCC 2007, Working Group 1.

[2] The concentration of carbon dioxide (CO2) in parts per million over the last 20,000 years reconstructed from Antarctic and Greenland ice cores and direct atmospheric measurements (red line). The grey bar shows the reconstructed range of natural variability for the past 650,000 years. From Fig.TS.2, IPCC 2007 Working Group 1, Technical Summary.

[3] Comparison between Caribbean shallow water (blue solid line) and deeper water (red dotted line) sponge delta13C records (inner left y-axis; delta13C is a measure of the 13C:12C ratio), delta13C of atmospheric CO2 (green crosses, outer left y-axis) and atmospheric pCO2 (circles, right y-axis, reciprocal scale; pCO2 is a measure of the CO2 concentration and temperature of the atmosphere). Atmosphere data are derived from Antarctic ice air inclusions and air measurements. Blue bar at the top shows the approximate duration of the Little Ice Age. Red bars mark the Spörer (ca. 1420-1540 A.D.) and Maunder (1645-1715 A.D.) sunspot minima. From Böhm, F., A. Haase-Schramm, et al. (2002). Geochemistry, Geophysics, Geosystems 3(3): doi:10.1029/2001GC000264, who give further details of sources and how the figure was constructed.

[4] The upper figure shows the positive trend of the CO2 concentration in the atmosphere since 1960 (expressed as the mass of carbon; 1 Pg = 1 billion tonnes). The lower figure similarly shows the CO2 emissions from fossil fuel combustion and cement production, and from land use changes. The shaded area is the uncertainty associated with each component. At any one time, the sum of the sources in the lower panel exceeds the corresponding figure in the upper panel because the difference is absorbed by oceanic and terrestrial sinks. From Le Quéré et al., 2009. Nature Geoscience 2, 831-836.

[5] Decreases in the infra-red radiation received by orbiting satellites in 1997 compared with 1970 caused by increased absorption of infra-red radiation by the named trace greenhouse gases. Dips below the zero brightness line indicate increased absorption where ‘Brightness temperature’ indicates the equivalent blackbody temperature in degrees Kelvin (From Harries, J. E., H. E. Brindley, et al. (2001). Nature 410 (15 March 2001): 355-357).

[6] Changes in temperature since 1850 over the land and the oceans relative to the average temperature. Each of the different coloured lines in each panel represents an independently analysed set of data. The data come from many different technologies including weather stations, satellites, weather balloons, ships and buoys. From Arndt, D. S., M. O. Baringer, et al. (2010). Bulletin of the American Meteorological Society 91(6): S1-S224.

[7] Comparison of observed global-scale changes in surface temperature with results simulated by climate models using natural and anthropogenic forcings. Decadal averages of observations are shown for the period 1906 to 2005 (black line) plotted against the centre of the decade and relative to the corresponding average for 1901 to 1950. Blue shaded bands show the 5% to 95% range for 19 simulations from 5 climate models using only the natural forcings due to solar activity and volcanoes. Red shaded bands show the 5% to 95% range for 58 simulations from 14 climate models using both natural and anthropogenic forcings. From IPCC 2007, Working Group 1, Technical Summary, Figure TS.22.

 

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