Those involved in climate research have been "revising" or "weighting" old data, and it so happens that this revising has emphasised warming in the last two decades as against temperatures experienced during a warm period between around 1000 and 1350 AD and in the 1930s. The issue is whether this revising was for objective scientific reasons - to account for example for a better understanding of differences in measuring techniques applicable to estimating temperatures at different times in the past (eg from tree rings) - or whether the hacked e-mails show that there has been an illegitimate manipulation of data done because those who did it thought it was "in a good cause", namely to strengthen the case for the saving of the planet. A sub-issue is the extent to which the collected data is still objectively reliable given this revision.
Some facts are a matter of accepted science by all those on both sides of the debate. It is indisputable that human activity has affected and will continue to affect the global climate. Everyone accepts that carbon dioxide at trace levels is a greenhouse gas, the release of which into the atmosphere by the burning of fossil fuels will increase global temperatures. The question is to what extent and whether it matters.
The problem here is that the impact of increases of carbon dioxide in the atmosphere creates instabilities: every mole of carbon dioxide that is released into the atmosphere causes through warming approximately an additional two moles of water vapour to be released, which has a more potent greenhouse effect than does carbon dioxide. In addition, as polar ice melts, the heat absorption of the earth increases (less solar radiation is reflected back into space) thus also increasing temperatures, and the methane trapped in frozen northern latitudes is also released to the atmosphere, which is itself a very potent greenhouse gas. Against that there are balancing factors the other way, namely that both the oceans and natural plant growth (and plant growth increases as levels of carbon dioxide increase) form carbon dioxide sinks; and increased cloud formation increases the reflection of solar radiation from earth. It is indisputable that a point would be reached at which the second will not be capable of accommodating the first and some form of runaway would take place. This tipping point would come at lower levels of fossil fuel consumption if we continue to cut down the rain forests, as the rain forests form a significant proportion of the earth's current carbon sink. There are other natural processes, in particular rock (silicate) weathering, which decrease levels of carbon dioxide, but the time scales for this are much too long to operate in the period we are concerned with.
All climate scientists agree that the ultimate fate of the earth will be thermal runaway, and the climate will become similar to that of Venus - temperatures well beyond 100C and so hot as to incapable of sustaining any form of conventional life. This will occur irrespective of the effects of mankind on the environment because the luminosity of the sun is increasing at a rate of about 8% per 1 billion years as it expends its nuclear fuel. The earth is now approximately 4.5 billion years old, and plant life in one form or another has existed for about 500 million years. In approximately 100 million years the indirect effects of rising temperatures on photosynthesis from increased solar radiation will cause most of today's plant life to die out (perversely perhaps, this will arise because silicate weathering from increased temperatures will by then have reduced levels of carbon dioxide to below those necessary for the main form of photosynthesis to take place). In 1 billion years' time the oceans will have evaporated and earth will begin losing some of its atmosphere; and by about 4 billion years' time complete thermal runaway will have taken place. Such life destroying thermal runaway will be later followed by the sun turning into a red giant in its final period of nuclear fusion about 7 billion years from now, during which the earth will be burnt to a crisp. The sun will then collapse and be extinguished (it will become a black dwarf).
Such extinction of life is a long way ahead viewed from our perspective (although a short time ahead in terms of the evolution of life on earth), and I entirely accept the argument that we do not want to hasten our demise, but as Private Fraser would have said, as a species "we're all doomed I tell ye"1.
Moving back to the present, it should also be noted that the earth has already experienced wide climate variation. We are at present in an ice age, that is a time when there are permanent ice sheets on both poles. To be more exact we are in an inter-glacial period of an ice age. There have been times (in fact the majority of time in the history of animal life on earth) when there have been no permanent ice caps. There have been other times when the ice sheets have nearly extended to the equator - the "snowball earth". This climate variation has been caused by variations in the luminosity of the sun (which as mentioned increases over time), the position of the earth and its orbit in relation to the sun and most particularly by the effect of the movements of the continents through plate tectonics on ocean currents.
Where we are
For the reasons mentioned above, in planning for the future we are looking at around the next 50 to 100 million years (a long time by human standards). After that, the bets are off from natural causes which we cannot avert. As I say, the question is whether human induced global warming (as opposed to long term solar induced global warming) matters when looking at that period. The problem we have is that we don't know for sure how serious those effects will be, but there certainly will be effects.
The two possible effects from increasing levels of carbon dioxide deposition in the atmosphere are:
- Warming that does not reach the point of causing a runway effect: this would involve melting of the polar ice caps causing a significant rise in sea levels, so flooding a substantial proportion of present land masses, and significant changes in rainfall and weather patterns in relation to the land which remains.
- Warming that gives rise to thermal runaway.
The current furore about whether global temperatures have decreased over the past decade is really a side issue, since short-term climatic variation is quite normal: there was a warm period about a thousand years ago, and a cold period about 600 years ago. The fact of the matter is that viewed from a longer term perspective, releasing carbon dioxide into the atmosphere will cause long-term rises in global temperatures, some polar melting and some changes in weather patterns. The latter two are being observed now. (Although global temperatures may not have been rising over the past decade, polar temperatures certainly have.) The question is how much of this will be caused and to what extent we can take it as a global society.
To give complete balance, the interglacial period we are now in will also at some point end. This will likely cause some re-glaciation of northern areas, and thus also population relocations and possibly war. But the time scales for that are much less pressing than those caused by global warming through consumption of fossil fuels and deforestation.
The fact of the matter is that levels of carbon dioxide in the atmosphere are increasing and this will have an effect. Future generations will rightly curse us if we fail to deal with it now; and it is getting urgent. It would be a great shame if any unscientific activities by those at the Climate Research Unit that may have taken place because they "thought it was in a good cause" - and we won't know whether there were or not until the inquiry at present under way reports - serves to cloud this truth.
1 Even if the human species escapes thermal runaway on the earth by establishing a human-friendly atmosphere on Mars and moving there, life on Mars would also be made unsustainable when the sun enters its red giant phase. Even if that is avoided by moving to another solar system, if one wants to be particularly morbid it should be noted that the second law of thermodynamics and the unavoidable increase in entropy over time give us a one-way ticket to nothingness, albeit in a very, very, very long time, even if other unpleasant events do not intervene before then such as the decay of fundamental particles, a big crunch or a big rip.