Obviously there is more than a little bit of controversy over the greenhouse effect and global warming. Is the World's temperate rising? Is this due to human activity? If it is rising what will be the future consequences? What can we do about them? What should we do about them?
The measures required to deal with global warming are very expensive. They have the potential to do great damage to the economies of countries, possibly creating mass unemployment and poverty. Even if they don't the restrictions on peoples activities involved could be quite irksome. Thus people want to be quite sure that there is a real danger before they make the sacrifices that are necessary.
On the other hand if global warming is real the consequences of not dealing with it are potentially catastrophic. The likely damage from global warming is far greater than the damage from an unnecessary attempt to deal with it. At the worst it could lead to the collapse of nations, to mass starvation, to enormous refugee problems, to wars and to huge damage to civilizations. And that is just the potential effect on human beings. The damage to other life forms could be even worse. We appear to be in a human generated mass extinction event. Global warming would aggravate this.
The trouble is proving that we have global warming and that it is anthropogenic is not straightforward. The phenomena involved are very complex and we do not and I believe cannot have any single easily understood piece of evidence that clearly says to everyone that global warming is real and humans are causing it. As well it is tangled up with other issues. All too many people are taking a position on global warming because that is the position that those with their politics take.
The scientists who first realized that there was a risk of disastrous warming caused by greenhouse gas wanted to get the message across to people who could do something about it - both people in government and the general public. To do so they oversimplified and they under emphasized the uncertainties of their conclusions. In other words they argued like politicians.
This succeeded in getting attention and prodded some people into action. It antagonized some others. They were annoyed at scientists who acted and urged others to act as if they were certain of their conclusions. And they could not reasonably be certain until recently. Some scientists were prematurely certain (Well as certain as scientists get.) Others believed that serious greenhouse gas forcing of global temperatures was far more likely than not and given the likely consequences action had to be taken. They believed (unfortunately correctly) that any show of uncertainty about their conclusions would be seized on by opponents as reasons not to act. But the gaps in their evidence and arguments were spotted and skeptics were able to get the ears of many of the public and many politicians and officials, not to mention industry leaders. Treating significant greenhouse gas forced global warming as a certainty had mixed results. But would frankly saying that we needed to act on incomplete information have led to results that were even as good?
They had under emphasized convincing industry leaders. Part of this was the belief that if they could convince authorities to pass and enforce the appropriate regulations and convince the public to change their habits then industry would have to fall into line. Part of this was pessimism. They believed that industry leaders were likely to indulge in wishful thinking about global warming and that it was a waste of time trying to convince them. Unfortunately there was some truth to this. As people with a lot to loose from the measures required to counteract global warming industrialists tended to look for reasons not to act. Finally but I think less importantly most public sector scientists tend to be slightly left of center and tended to disregard industry owners and management. But this was a mistake. Industrialists were capable of understanding the arguments put up by greenhouse skeptics and of using them to lobby government.
And then there were the greenies. Most of them are opposed to some aspects of modernity. The greenhouse effect became another club to bash industry, urbanization and modernity with. There is a return to the past streak in the green movement. We cannot return to the simpler life that they espouse. There are too many people now for this to be possible. And most people only want part of the changes that the greenies want. They do not want to give up their luxuries and are turned off by the self-righteous asceticism of the greens. The support of the greens for measures to combat global warming got the backs of many people up. The trouble is I think this is an area where they are partially right.
The greenhouse skeptics have done something worse. They have argued like lawyers. Rather than looking for the most likely explanation many have looked for reasons to believe that greenhouse warming is not real. Some of this is quite reasonable antagonism towards the greens. Some of this is understandable and often justified antagonism towards regulators. Some is wishful thinking.
Some skeptics want simple overwhelming arguments that anyone can easily understand. They are uncomfortable with cases that depend on many lines of argument where none of them are individually conclusive but to disbelieve the lot strains credibility. Unfortunately Nature has not cooperated. We have very complex systems and very noisy data. The proof of human generated global warming depends on the convergence of evidence.
To establish that there has been anthropogenic global warming we have to answer three questions. Have greenhouse gas levels increased as a result of human activity? Have global average temperatures risen? Has the increase in greenhouse gas levels caused the temperature increase? As far as I can tell the answers are yes, yes and partially.
Air bubbles trapped in glacial ice cores have been analyzed. These samples go back several hundred thousand years. The current CO2 levels are the highest found in the whole period. There has been roughly a 30% increase in CO2 levels over the past 150 years. The carbon isotope composition shows that three quarters of the increase in CO2 has come from the burning of fossil fuels and the rest from land use changes (mostly deforestation). Carbon from organic sources is depleted in C13 and C14. Carbon from fossil fuels has almost no C14. I cannot see any reason to doubt the claim that human activity has led to major increases in CO2 levels.
Finding trends in global average temperatures is much more difficult. The data is very noisy, there are many local complicating factors such as the effects of nearby urban areas and we have difficulties with some of the upper atmospheric measurements. One has to combine data from measurements across the World and over decades. This is a difficult and very complex procedure. Nevertheless it definitely looks as if average temperatures are increasing.
Terrestrial surface temperatures have shown an increasing trend with time. It has been suggested that this could be due to increasing urbanization leading to an increase in the urban heat island effect on instruments. Analysis of the data has shown the urban heat island effect to be too small to account for the trends.
The strongest piece of evidence is the recently completed work on ocean surface and upper layer temperatures. This showed temperature increases with time. Because of the sheer thermal inertia of the oceans data will be less noisy than terrestrial data and any trends observed are likely to be reflected elsewhere.
There were doubts surrounding tropospheric temperature readings. The expected trend did not show up. The data has recently been reexamined and systematic instrument biases were found. Once these are allowed for the data shows the expected increasing trend.
Stratospheric temperature readings have shown a decreasing trend as would be expected if temperature trends were driven by greenhouse gas forcing. (The CO2 blocks heat that is being re-radiated from Earth.)
The warming trend is showing up in too many forms. I think we can definitely say that the world has warmed over the past century.
Showing that the world has warmed and that greenhouse gases have increased does not prove that the second has been a major contributor to the first. For thousands of years there has been decade to decade and century to century variation in temperatures. The question is whether the current warming is due to the natural causes that have led to previous warmings or whether it is due to anthropogenic effects or whether it is due to both natural and anthropogenic effects.
The difficulty in connecting greenhouse gas increases and global warming is that we cannot do controlled experiments on the climate. We have only one world available to us and we can only observe what is happening. To understand what is going on and to make predictions computer models of climate have been created and experimented with. Of course these models are approximations and simplifications. Nevertheless they are what we have and we have to use and improve them. We have to create models which incorporate enough of the variables driving temperature change in a realistic enough manner. These can allow us to understand what is going on and to make useful predictions. We can treat a model as an adequate approximation to reality when its predictions match up adequately well with observations. We show that greenhouse gases are responsible for at least some of global warming by showing that observations can be explained if significant greenhouse effects are included and we cannot explain the observations if we do not include them. Of course the models are as simple as is compatible with explanatory power.
Increases in global temperatures can be explained in three ways. There can be an increase in the heat input i.e. solar variation. There can be changes in the ability of heat to move through the atmosphere The main causes of variation in this as far as we can tell are gases and aerosols either of human or volcanic origin. The final possibility is changes in heat flux from the Earth. This would be either heat of geothermal origin or heat that had been stored in the ocean and was now being released.
The current models focus on changes in heat input to the Earth and on changes in atmospheric properties. We can rule out a geothermal origin for the additional heat. Geothermal heat would be localized and it would show up in underground temperature readings. Ocean currents do play a big part in moving heat around the world. Cycles in ocean currents and sea surface temperature could be involved in decade to decade variation in hurricane frequency. Storage of heat in the oceans and its release after a delay of decades or centuries would probably require implausibly complicated processes. I have not seen it suggested as an explanation for the current episode of global warming and if any does suggest it they need to come up with suitable model.
The earlier models tried to explain all of the increase in temperature by changes in atmospheric composition. They were too simple and did not give good predictions. Current models include atmosphere composition changes of both human and volcanic origin and they include variations in solar output. They give a good fit to the observations and should be accepted as explanations and used for predictions until better models are created.
Volcanism explains many short term drops in temperature. Solar variation explains a large proportion of the increase in temperatures in the first half of last century. Future model refinements will give us a better idea of just how much. Anthropogenic greenhouse gas forcing also seems to be involved in that period. Solar variation cannot explain the temperature increases of the past three decades. In fact in the absence of greenhouse gas forcing one would have expected declining temperatures. The temperature increases in the past three decades appear to be of human origin.
Also I do not see how solar variation can explain stratospheric cooling while we have increases in surface and lower atmosphere temperatures. Greenhouse gas forcing would have exactly this effect.
It is the consensus of scientists working on climate that the Earth is warming as a result of human actions. There are differences of opinion over just how much temperatures will increase if nothing is done to arrest the increase in greenhouse gases. Most believe that over the next century we will have an increase in temperature greater than that in last century. It is no good attacking ideological biases. While most public sector scientists are a bit left of center I do not think that this has had a significant effect on their opinions on global warming. Science has good error correcting mechanisms. Most scientists are more interested in finding out what is going on than in pursuing an ideological agenda. In fact ideological agendas are usually driven by whatever subject they are interested in. Of course this does not make their proposed solutions to problems right even if their analysis of the problem is.
Thus I believe it is far more likely than not that we are in a period of rapid global warming mostly driven by anthropogenic increases in greenhouse gases. The consequences of not acting if greenhouse gas forcing of global temperatures is real are much worse than the consequences of acting if it is not real. The more we delay the harder it becomes to take corrective action. We are already nearly as certain as we are likely to be in the near future. By the time in a few decades time that we are significantly more certain great damage will have been done.
Environmental degradation in all its forms is a less urgent problem than the war against Islamic terrorism. But it is still urgent and in the long term it is more important.
I am appealing to people to look at the facts. Yes, the greens are using it as a stick to bash free enterprise. But even some them are facing reality now and seriously examining nuclear power. Yes, some sacrifices will have to be made to minimize environmental damage. If we continue to indulge in wishful thinking about global warming we will rob all generations to come. If you say you are still skeptical, well what would you see as acceptable proof and is it possible to get it before major damage is done.
I am not going in detail into what I think we should do. I think the problem is manageable. Some of it is technological improvements and some of it is changes in our behavior. We have to accept that we have serious environmental problems and must do something about them. Just as we have to accept that we are in a war against a totalitarian movement and have to win it.