Home » Issue 8 » What do we know about climate? Investigating the effects of anthropogenic global warming
What do we know about climate? Investigating the effects of anthropogenic global warming
Submitted by rau on 29 April 2008
When protons from GCRs collide with the nitrogen-14 (seven protons plus seven neutrons in the nucleus) in the air, carbon-14 is created (in addition to other isotopes such as beryllium-10) through a nuclear reaction:
14N + p → 14C + n
This means that carbon with a low isotope carbon-14 ratio must come from deep in the ground, out of reach of cosmic rays.
Furthermore, the ratio of O2 to N2 has diminished. This is expected from the increased combustion of fossil fuels, in which O2 combines with C to form CO2. The oceans have also become more acidic, leading to an increase in CO2 levels in both the atmosphere and the oceans.
According to the Fourth Assessment Report (AR4) from the Intergovernmental Panel on Climate Change (IPCC), the global mean temperature, estimated from thousands of individual thermometers scattered around the globe, has increased by 0.74 ± 0.18 ºC over the past 100 years, and appears to be rising still. Some satellite-based studies have also reported changes in the spectral characteristics of the heat radiated from Earth, in line with an increased greenhouse effect. Bore-hole measurements from below the surface can also be used to infer temperature changes, and these too indicate that there has been a warming.
The global mean sea level is increasing, both due to the fact that warmer water has greater volume and because glaciers have melted. It has also been documented that most glaciers worldwide have retreated since the end of the 19th century. The sea-ice cover in the Arctic has diminished substantially since satellite measurement began, and the snow extent has decreased too.
There is also evidence from the hydrologic cycle: signs of more frequent intense downpours, and changes in the river discharge and rainfall statistics.
Additional reports on biological responses fit into the picture of a global climate change. Changes in the tree line, tree ring widths/density, corals, sea-bottom sediments and stalagmites bear witness to how the climate has varied in the past. Compared with these indicators, the present warming seems to be exceptional for at least the past 1000 years.
Those who dispute the notion of an AGW, popularly called climate sceptics, have argued that global warming is a consequence of changes in the Sun. But modern measurements of cosmic rays, sunspots and other indices used to describe the state of the Sun suggest that it has not become more active since the 1950s (see graph).
Changes in land surface may also play a role by changing the way the planet reflects light as well as the exchange of moisture and energy between the surface and the atmosphere.
Some sceptics argue that global warming is an illusion due to the effect of urbanisation. However, this does not explain how most of the world’s oceans have warmed both near the surface and at depths – where there are no cities. Nor does it explain why the greatest warming has been observed in the Arctic, Alaska and Siberia. Besides, the urban influence on the temperature record has been studied and taken into account when estimating the global mean.
Another argument used by climate sceptics is that satellite measurements of the temperature in the atmosphere do not show similar warming as on the ground. This discrepancy was due to errors in the analysis of the satellite data; the trends in the free atmosphere have now been reconciled with the surface data.
Others have argued that our atmosphere is already opaque to infra-red radiation and is hence saturated, so any additional CO2 will not have much effect. However, one only has to look to Venus to see that the atmosphere doesn’t saturate that easily. One can also show this theoretically. Besides, it is not only the effect of CO2 that matters, as there are important feedback processes which may amplify (such as retreating sea ice, or air moisture) or dampen (low clouds, for example) the response to a changing greenhouse effect.
But hasn’t the climate always been changing? There are scholars who argue that our climate has a naturally cyclic behaviour. How do we know that the present warming is not just a part of a natural cycle, such as a rebound from a short ice age?
We can deduce from the laws of physics that the mean temperature does not just change spontaneously, as heat must be conserved. In the past, something in particular caused the variations – be it changes in the Earth’s solar orbit, atmospheric composition, solar activity, volcanoes or landscape changes. None of these factors, except for changes in GHG concentrations, can explain the current warming. Even changes to the system itself, such as El Niño Southern Oscillations, may produce some variations, but these tend to be weak compared with the changes due to external forces. And regardless of cause, there is always a physical explanation for the changes, be it external or internal.
Can we really trust global climate models (GCMs)? GCMs are not perfect, but they are still the best tool available for making projections for the future. A GCM may be thought of as a jigsaw puzzle, where the large picture emerges from small pieces put together in a consistent and organised manner. GCMs incorporate everything we know about the climate system in terms of physical laws and empirical data, and provide a comprehensive picture through the means of numerical methods on large computers. Some equations describing the processes cannot be solved exactly, but approximations nevertheless provide a good representation.
Projections for the future
So what can we expect from an AGW? The scientific findings published in peer-reviewed scientific journals have been collected and assessed in the IPCC’s report, which then presents the main picture for the future.
The hurricane season of 2005 produced an unprecedented number – at least in modern times – of tropical cyclones in the Caribbean/North Atlantic, some of which resulted in substantial casualties and severe damages. Will there be more frequent or more powerful hurricanes/typhoons when the world is warmer? And are we now witnessing a trend in tropical cyclone activity? At present, we cannot be sure, although there are some indications that the potential for storm intensity may rise, and that there has been an upward trend in the activity associated with the more intense tropical cyclones over some ocean basins.
Work within ESA’s Data User Element (DUE) currently includes studies into condensation trails and any effects they might have on climate change.
In addition to the scientific issues surrounding AGW, there are clearly ethical aspects too, such as those associated with the realisation that rich countries bear most of the responsibility for increased emissions of GHGs but are the least affectedw1. There are also energy considerations, and the question whether renewable sources of energy can replace fossil sources. Furthermore, economic considerations and political choices concerning climate change are closely related to energy options and greenhouse gas emissions.
The climate debate may indeed be one of the most profound issues of our time. It would be a shame if the broader public could not participate in this debate due to lack of understanding. Hence, it is important that schools teach students about climate and climate change, and that their information is accurate and up-to-date.
w1 – For a discussion of the politics of climate change see www.OpenDemocracy.net
Everyone has heard about climate change. The subject is frequently addressed by the media, but the information given is often incomplete or biased by political views. As science teachers, we have to provide students with correct information and instruments to develop critical attitudes based on facts and aimed at active citizenship.
This second of two articles by Rasmus Benestad is very useful because it presents the topic clearly and objectively, addressing the evidence for anthropogenic climate change.
I recommend this article to science teachers who are willing to update their knowledge and to secondary-school students interested in the facts and scientific evidence at the heart of this debate. The material is also particularly suitable for classroom discussion and for an interdisciplinary approach to environmental education in secondary schools.