The evidence I’ve found so far strongly suggests the contrarians are wrong. Which is a real shame, because the contrarian argument is winning as long as there is no strong worldwide move to cleaner energy. Since I don’t want disaster, I would really like them to be right. Unfortunately, nature has no concern for our preferences.
One of the problems with understanding why adding CO2 to the atmosphere drives climate change is there isn’t a simple equation that shows how the physics works. To calculate the actual effect requires taking into account the overall complexity of the planet, including the nature of the atmosphere that not only varies in density with altitude but also composition – water vapour for instance is not only highly variable in its geographic spread but the range of altitudes over which it is present.
If you want a comprehensive understanding of the greenhouse effect and how it applies to a whole planet, you have little option short of reading a textbook (e.g. RT Pierrehumbert’s Principles of Planetary Climate).
Nonetheless there are plenty of lines of evidence that make the case even if you don’t have a deep understanding of physics and calculus. Here are some.
Faint Young Sun4-billion years ago, the sun’s intensity was only 70% of its current energy output. That would not have been sufficient under today’s conditions for the oceans to be liquid, yet the geological evidence is that there were liquid oceans that long ago. During that time, there was no life as we know it, and greenhouse gas concentrations would have been a lot higher than they are now. Greenhouse gases are the best explanation for liquid oceans at that time, though there is a lot of uncertainty given how long ago this was, and the limited range of geological data. Even so, we can be reasonably sure that a CO2 level ten times or more the current level would have had a very significant warming effect, and that coupled with other greenhouse gases could have made the planet warm enough for liquid water.
Snowball EarthSeveral times in the distant past, with the last such event about 630-million years ago, the Earth froze over completely, almost to the equator. Under these conditions, CO2 cannot be drawn down from the atmosphere. There is unlikely to be sufficient plant life – even phytoplankton – able to consume CO2, and natural chemical processes that draw down CO2 require liquid water. In a snowball earth, CO2 in the atmosphere gradually increases until there is enough of a greenhouse effect to start melting the ice. This CO2 builds up from emissions from volcanoes, normally a minor effect offset by weathering, where rocks high in calcium dissolve in carbonic acid (CO2 in water) to form carbonate rocks such as limestone. Weathering requires liquid water, so the chemistry required stops if the planet ices over completely. Consequently any CO2 vented by volcanoes that escapes the ice cover into the atmosphere stays there. Without a greenhouse effect, the ice would not melt because ice has a high albedo (fraction of incident light reflected), and any region covered in ice reflects a high fraction of incoming solar energy back to space. Absent an increased greenhouse effect, a planet can only shed such a complete ice cover after a wait of millions – possibly billions – of years for the sun to become warmer.
A snowball earth can eventually end in a return to a warmer world as a result of the steady accumulation of CO2 in the atmosphere, leading to sufficient warming to melt the ice back to the polar regions – some simulations show this taking as little as 2000 years, a sharp contrast to waiting billions of years for the sun to warm up.
|Milankovitch Cycles and Climate. The top three graphs|
derive orbital parameters and the fourth (from the top)
shows resulting variation in northern hemisphere solar
energy. The bottom two graphs are indications of past
climate variability that can be related to the fourth
graph. For more detail, see WikiPedia.
In this scenario, CO2 is acting as a positive feedback, an amplifying effect, rather than as a primary driver of climate change.
Reverse the scenario: if a change in the Earth’s orbital parameters causes cooling, the oceans dissolve more CO2 and once again CO2 amplifies the change.
Other feedbacks include a change in ice (less ice means less heat is reflected to space) and increased water vapour (a strong greenhouse gas) but these effect are insufficient to explain the temperature swings.
PETM in context. In geological time the PETM event is for
practical purposes instantaneous – yet happened about 50
times slower than current climate change. Source: WikiPedia.
|Planetary temperatures. Note that though Mercury|
is much nearer the sun than Venus, Venus has a much
higher average surface temperature. Source: NASA.
- distance from the sun (or in effect the amount of incoming solar energy)
- albedo (the fraction of incoming energy that is reflected)
- atmosphere (ability of the atmosphere either to reflect energy back to space, or to slow the rate of energy flow out to space)
SummaryAll these lines of evidence are a small part of the picture. If here is indeed a very low climate senstitivity to increased CO2, all of the theory explaining these events would be wrong. So contrarians not only have to explain why prediction of future climate change are wrong, but also have a reasonable alternative explanation for all these lines of evidence.
The current theory of climate, which takes into account variations in solar output, changes in the atmosphere and various modes of redistribution of energy around the planet, has been well tested in several ways, including modelling the paleoclimate, forecasting climate change then checking as new data comes in, and hindcasting (testing predictions against known data). In addition, there are other lines of evidence like shifts in the range of temperature-dependent plants and animals.
The current theory of climate started from understanding the shifts between ice ages and temperature on other planets of our solar system. The theory continues to be refined as more data becomes available and more computational power is available for more complete models. So far, the major effect of improved models has been refinement rather than refutation. Every now and then a contrary result appears, but it does not stand up to rebuttal.
Every now and then, a model prediction turns out to be incorrect. It would be surprising were that not the case, with such a large-scale, complex system to model. Pointing at such flaws as indicating there is no problem and we should continue with business as usual is silly – like a morbidly obese patient deciding not to lose weight when a heart attack scare turns out to be a false alarm.
Without a strong greenhouse effect, the Earth could not have had liquid water when the sun was only emitting 70% of its current energy. Without a greenhouse effect, the Earth could not have escaped a snowball state. The greenhouse effect is required to amplify the effect of Milankovitch Cycles, otherwise orbital variations are insufficient to explain deep difference between glacial and interglacial climate. In all these cases, CO2 has to play a prominent role. There is no other greenhouse gas that can vary on a sufficient scale to make a difference. The kind of very rapid climate change – particularly warming events – that has triggered some of the biggest mass extinction events can also only be explained by the greenhouse effect. Finally, the greenhouse effect has been thoroughly studied for other planets and nothing else can explain the extremely high surface temperature of Venus.