Climate Change Lines of Evidence

Much of the contrarian position on climate change relies on question the sensitivity of the climate to varying CO₂ content of the atmosphere. The mainstream science says that doubling the CO₂ concentration should add about 3°C, with some variation depending on the starting conditions (e.g., the amount of ice at the start). Contrarians claim that climate sensitivity to increased CO₂ is much lower, perhaps at most 1°C per doubling – and some even claim the greenhouse effect is already saturated, meaning more CO₂ will have no effect. All these contrarian claims are simply claims without evidence to back them up.

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 Sun

4-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 Earth

Several 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.

Milankovitch Cycles

Serbian scientist Milutin Milanković, through laborious calculations, demonstrated that variations in the Earth’s orbit over millions of years corresponded to movement in and out of ice ages. Physics calculations of the change in incoming solar energy cannot account for the temperature swings needed to change between states of kilometres-thick ice caps and no ice over continental-scale regions. On the other hand, we know that the oceans’ capacity for dissolving CO₂ is temperature dependent. As temperatures increase, the oceans release CO₂. This increases the greenhouse effect, and hence amplifies a temperature increase. The opposite applies when temperatures are decreasing.

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.

Mass Extinctions

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.

Our planet has undergone several mass extinctions where a large fraction of life was eliminated. These mass extinction events generally follow a massive planetary climate change. In some such events, the change has been a rapid temperature rise, preceded by a rapid increase in greenhouse gases. Good examples are the end-Permian extinction (the biggest of the mass extinction events, about 253-million years ago) and in the Paleocene–Eocene Thermal Maximum (PETM). The PETM is the more recent of these two events, and hence a bit easier to study in detail. The evidence if that temperatures increased by 6°C over about 20,000 years. Such an increase could be caused by a quadrupling of CO₂. It’s sobering to consider that such an increase over 20,000 years caused a major extinction event, with an average increase of 0.003°C per decade, compared with the current rate of increase of about 50 times faster.

Other Planets

Planetary temperatures. Note that though Mercury
is much nearer the sun than Venus, Venus has a much
higher average surface temperature. Source: NASA.

The temperature of a planet is dependent on three major variables (with other factors like oceans contributing):

• 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)

The planets from the sun outwards should be increasingly warm, if the first effect is the only one. This is mostly the case, but Mercury has a lower average surface temperature than Venus, despite being much closer to the sun (57,910,000 km, versus Venus at 108,200,000 km). Much modern climate science derives from study of the temperatures of other planets. Venus has a dense atmosphere mostly containing CO₂, so it has an extreme greenhouse effect. Other planets have different atmospheres (Mars’s atmosphere, for example, is also mostly CO₂ but much less dense than the Earth’s) , and modelling these variations is a starting point in the theories that today are used to model the effect of increasing CO₂ on our planet’s climate.

Summary

All 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.