I want to formulate a basic case regarding global warming based on some study I've been doing in response to global warming denial. In addition to the basic case I want to address some common skeptical claims.
I think a good place to start if you are trying to formulate an opinion about this is to look at the strength of the scientific consensus. Some of this is complicated and most of us aren't experts. Relying on experts is rational as a starting point. What do they say?
The scientific consensus is pretty overwhelming. The Intergovernmental Panel on Climate Change (IPCC) was a massive effort to assess the scientific consensus. It concluded that global warming is real, the recent spike is the result of human emission of greenhouse gases, and that it represents a serious threat to human kind as well as other living species. No scientific body of international standing rejects these conclusions. Studies show that 98% of climate scientists accept the IPCC conclusions and in fact the greater the level of expertise (as measured by the number of papers published) the more likely the scientist was to accept the IPCC consensus.
So why believe that global temperatures are rising? Because multiple peer reviewed studies show that it is. Take a look at this chart of global temperatures based on multiple studies.
You might ask, what are these based on? How can one know what the temperature was 2,000 years ago? The answer is that there are various proxies that can be used to attempt to answer that question including tree rings, coral, ice cores, and glacial records. These are imperfect of course. And the error bars generally enlarge the further away you get from the present day. But these various studies based on various proxies tend to converge on a similar conclusion.
The first historical temperature reconstruction produced a graph like this and it was referred to as the hockey stick. It was used by the 2001 IPCC Report, and, though the creators of it made obvious reference to the uncertainty associated with it, the IPCC literature presented it in a graphic without error bars. Global warming skeptics were outraged and aggressively attacked the scientists that produced the graph, even though the scientists presented the results tentatively and with generous error bars. It rose to the level of Congressional hearings. Wikipedia has a summary here. The whole Wiki entry is worth reading, but in summary the skeptics have made several runs at discrediting the chart. The first ended in disgrace (Saul and Baliunus). Next was an effort by a guy named Stephen McIntyre, which apparently did provide some decent correctives, but didn't change the overall conclusions. More recently a Koch funded study has tried again and failed (as I mentioned here). At least to this point. Meanwhile over a dozen peer reviewed scientific studies have been published confirming the general accuracy of the original study.
So what about CO2 concentrations in the atmosphere? Here is a historical chart.
Makes sense. You take massive amounts of hydrocarbons out of the ground and release the CO2 to the atmosphere and you get an increase in the concentration of CO2 in the atmosphere.
But why believe this is the cause of warming? There are a number of good reasons.
Here's one reason explained in some detail, but I'll offer the quick version. CO2 levels have increased dramatically relative to the past several hundred thousand years and CO2 absorbs infrared radiation. There's no debate on that. But here's the key. Outgoing IR radiation can be directly measured. The amount of outgoing IR radiation has dropped at precisely the frequency at which CO2 absorbs IR radiation. That means this radiation is being absorbed rather than released to outer space. Effectively it is like putting on a blanket.
Another reason is that the stratosphere is cooling while the troposphere is warming. That is, the very top part of our atmosphere has been cooling while the atmosphere closest to the ground is warming. View the surface and upper air temperature historical plot here. That's pretty bizarre, right? What can account for that?
It was a long ago predicted consequence of warming due to man made CO2 emissions (see that prediction here). Go here for a good description of the causes. I don't think I can summarize it any better than it is done at that link.
These are not the only reasons. Several more are discussed here. Once again, in the scientific community the consensus is strong.
There are legitimate scientists that dissent. There are two main categories to the objections. The first is the claim that solar forcing explains the recent warming. The second is that the recent warming can be explained by what is known as the Cosmic Ray Flux (CRF) and its effect on cloud formation. A brief overview of these views and discussion of why they are by and large rejected in the scientific community is offered in this Youtube video.
So now, on to some of the questions that arise from the right wing disinformation effort.
Didn't scientists predict a global ice age back in the 70's?
The short answer is no. Time magazine had an article that made this claim but failed to provide a source. Surveys of the scientific literature revealed that at the time about two thirds of the experts expected global warming and a third expected global cooling (not an ice age). You'll notice based on the hockey stick graph that temperatures had been falling prior to the 70's. This was due to pollution from coal fire plants. There was debate about which effect would dominate. Would it be the cooling from the aerosols or the warming from the greenhouse gases? It's impressive that despite the history of cooling the majority of experts still expected warming and they were right. Since then there has been a lot more good research and the debate has been resolved. The warmers were right. For an overview watch this video.
Hasn't CO2 concentration changes lagged temperature rise in the past by 800 years? Doesn't this mean the rise in CO2 in the atmosphere is an effect of warming, not a cause?
Often CO2 rises have lagged the temperature increase, but not always. There are a number of different mechanisms that can drive a temperature increase, including the earth's orbital variation, volcanoes, solar irradiance, meteorites, possibly cosmic rays. In the present case we know it is greenhouse gases because scientists have managed to rule out alternative causes and because we are observing the predicted effects of temperature rise through artificial greenhouse gas emissions.
What causes the CO2 concentration changes to lag the temperature changes? Let me explain with an example using the earth's wobble as the dominant forcing mechanism.
The angle of the earth's axis to the sun varies from about 24° to 21°. When the angle is 21° there is less direct sunlight in the polar regions and ice forms there. Imagine what would happen as the tilt shifts towards 24°. More direct sunlight on the cap melts some ice at the cap. Melted ice converts to water. Water absorbs more heat than ice. Ice reflects radiation. So now with more water and less ice the earth as a system absorbs even more heat. That heat melts more ice and you create a positive feedback. Runaway warming and ice melting.
CO2 is trapped in ice. So as ice melts CO2 is released. This is why the CO2 concentrations lag the temperature change.
This process can reverse. As the earth's axis heads back to 21° we again get the freezing of the polar ice caps. Once again you go from a state where water is absorbing radiation to ice reflecting the radiation. This is a cooling positive feedback. The additional ice and snow can trap CO2 and methane from escaping to the atmosphere, once again creating a positive cooling feedback. The CO2 drop in the atmosphere once again lags the cooling.
The real concern is not the temperature effect of CO2. It's that the slight warming caused by CO2 can create a positive feedback due to the ice and other effects. At that point you can have runaway warming that can't be stopped even if you stop producing CO2 artificially.
What about the hacked emails? Don't these show a conspiracy within the scientific community to mislead the public?
Absolutely not. Multiple independent studies have confirmed there was no wrongdoing. For a fun video explanation of part of the issue, go here.
Isn't the sun the dominant climate forcing mechanism?
Of course. Without the sun there would be no climate. And solar output does vary. Approximately every 11 years the sun's polarity flips, and this results in altered solar output. But the conclusion in the scientific community is that recent solar changes cannot account for the rise in temperature we've experienced. The sun's output has been quite stable, following it's 11 year cycle regularly. Yet temperatures are rising. Why? The consensus of the experts is the greenhouse gases.
In times past CO2 concentrations were much higher. Today they are much lower. Why wasn't it a problem then?
Because the concern is not strictly the CO2 concentrations but the combined effect of the various temperature forcing mechanisms. Take the Huronian Glaciation. About 2 billion years ago the entire globe was covered in ice. Remember what I said about the feedback mechanism. An ice covered globe reflects solar radiation. That means when the entire globe is covered in ice solar output could increase a lot and yet you still may not be able to extract yourself from the snowball earth condition. That solar heat is just not absorbed.
And the reality was the sun's output was in fact much lower then than it is today. Solar heat was about 75% of what it is today. How did the earth ever come out of the snowball condition?
The answer is CO2. Volcanoes continued to erupt. When they did they emitted CO2. And there was no vegetation to extract the CO2 from the atmosphere. It took a really long time, but finally the concentration was high enough (about 1000 times what it is today) that ice started to melt. Once again the positive feedback kicked in. The ice melted rapidly. After it was all melted the earth continued to warm. Even with the sun producing a mere 75% of the output it is making today the earth was still much warmer thanks to the greenhouse effect. Vegetation began extracting CO2 from the atmosphere and though solar output increased the temperature on earth began to drop.
But why do I hear so much criticism about global warming in the media and from the right wing think tanks? These are smart people. They must know something.
Of course they are smart people, and they may know what the scientific community knows. But what is there job? Informing you? Or serving their owners and customers. I work for a corporation. In my professional capacity who do I serve? The owners and customers. Is Rush Limbaugh any different? Is the American Enterprise Institute any different? Limbaugh works for a mega corporation. His customers (advertisers) are mega corporations as well. He's going to spin tails that serve their interest. AEI is going to funnel money from Exxon to climate science denial efforts. Why? It serves their profit agenda. AEI's job is to confuse you for the interest of profit. They do it for economic matters. Why wouldn't they do it for environmental matters? You should expect spin and lies from them. That's what they deliver. Stop listening to them if you want to be informed.