Temperature increases over the past century are explained by the Urban Heat Island Effect – that is, by urbanisation – in that cities are ‘darker’ and therefore reflect less heat than rural areas.  Fact: There is a little bit of truth to this, but the impact of the Urban Heat Island Effect on global temperatures is minuscule compared to that of CO2 and methane.

We don’t know what proportion of the CO2 in the atmosphere is natural and what proportion was emitted by human activity. The vast majority of it could just be natural.  Fact: A molecule of CO2 from burning fossil fuels does not contain the isotope Carbon-14, as this decays over time (indicating that the carbon has been in the ground for millennia). CO2 from fossil fuels contains only Carbon-12 and Carbon-13. It is therefore not difficult to determine what proportion of the CO2 in the atmosphere is a consequence of human activity. [Interestingly, the existence of C-14 allows us to date plant material up to 60,000 years old, since C-14 has a half-life of about 5,700 years. This process is known as ‘radiocarbon dating’. It is regularly used in archaeology.]

The main greenhouse gas isn’t CO2, it’s water vapour. It is responsible for about half of all global warming, yet there is no way we can exercise control over it. As such, there is nothing we can do about most of the warming that is taking place.  Fact: Water vapour is an important greenhouse gas, but its level is determined by atmospheric and sea surface temperatures. Studies show that for every 1°C of warming, there is a 7 percent increase in water vapour. This creates a powerful feedback effect that further heats the atmosphere and the oceans. In other words, we do have control over water vapour, but indirectly, via our emissions of CO2. The only way to stop water vapour from increasing is to stop emitting CO2 and other greenhouse gases.

The amount of CO2 released each year by humans is only about 4 percent of all global emissions. The other 96 percent occurs naturally. So, our impact is very small.  Fact: The biosphere does emit and absorb CO2 all the time. Those two processes cancel each other out, leaving the system in equilibrium. However, the 4 percent emitted by human activity every year has built up over time and now represents 34 percent of all the CO2 in the atmosphere (up from 280 parts per million in the 1880s to 427 parts per million today). Nature has actually been resisting that increase, by absorbing up to 60 percent of what we’ve been releasing – mainly into the oceans. This process is keeping atmospheric CO2 levels below where they would otherwise be, but it is increasing ocean acidification significantly. Acidification is having its own detrimental effects on the biosphere, threatening fish species and contributing to coral bleaching.

Studies show that changes in the level of CO2 follow changes in temperature, rather than precede them, so they can’t be the cause of global warming.  Fact: Changes in temperature do trigger changes in the level of carbon dioxide, by causing the oceans to emit more or less CO2. This in turn changes the Earth’s temperature, by increasing or decreasing the absorption of heat in the atmosphere (which in turn changes the level of water vapour – a major greenhouse gas). In other words, global temperature and CO2 levels lead and follow each other in mutually reinforcing cycles. It is this process, over a period of millennia, that results in ice ages and thaws. 

Scientists can’t prove that emissions of CO2 and methane are the cause of global warming. Until they can, we should resist taking action. Fact:  It’s true that scientists can’t prove the link between greenhouse gas emissions and temperature changes, but that is true for many areas of science. The theories of gravity, relativity and evolution cannot be proven. Nor can the link between smoking and cancer. That doesn’t mean we shouldn’t accept these theories as true. We accept them because they provide convincing, testable explanations of observed phenomena. We believe in the theory of gravity, for example, because there is no other way to explain why objects fall to the ground when dropped. Even so, it is still theoretically possible that something else is responsible. For example, angels could be sitting on the objects, pushing them down. That’s what people believed in medieval times. But no modern scientist would accept that view now; it simply doesn’t fit the facts.  Hence, while we cannot prove that greenhouse gases are the cause of global warming, we can trust the scientific community when it says that they are. This is because they provide the only explanation that fits with observed reality.

Global warming is the result of volcanic activity, not the burning of fossil fuels.  Fact: Volcanoes have caused global warming in the past, when they erupted en masse and emitted vast quantities of CO2, but this has not happened for many millions of years. The level of CO2 emanating from volcanoes each year now is about 1 percent of that emitted by humans – not enough to have any impact on temperatures. Moreover, present day carbon emissions contain relatively little of the isotope Carbon-13, meaning they had their origin in plants rather than volcanoes. [Plants preferentially select Carbon-12 rather than Carbon-13 during photosynthesis.] In fact, volcanic activity in modern times has a cooling effect, not a warming one, as it increases the particulate matter in the atmosphere, thereby increasing albedo (the reflection of the sun’s rays). Major eruptions can reduce the Earth’s temperature by as much as half a degree for a period of several years, but these effects are temporary. The last major eruption that had an impact on global temperatures was that of Mount Pinatubo in 1991

Temperature changes are caused by changes in the Earth’s orbit, not by CO2 Fact: Over many millennia, the Earth’s temperature changes significantly. Those changes are set off by variations in the planet’s orbit, tilt and axial rotation (all of which are affected, in turn, by the gravitational pull of the larger planets). The change from an ice age to a thaw and back again is known as a Milankovitch Cycle. What happens is that once every 100,000 years the Earth’s orbit, tilt and rotation line up in such a way as to significantly change the amount of solar radiation reaching the planet. This in turn sets off a feedback loop that induces an ice age or a thaw. It takes between 5,000 and 20,000 years for the Earth’s temperature to make the change. It then remains stable until the next correlation of orbit, tilt and rotation occurs. The Earth emerged from the last ice age around 12,000 years ago and is not due to enter another for about 70,000 years.  Milankovitch Cycles are not the cause of our current global warming.   Ice ages and thaws over the last 800,000 years [Source: Wikimedia Commons]