In the simplest possible terms, explain global warming.
Global warming is the increase in the average temperature of the planet that has occurred over the last 100 years, and especially during the last few decades. It is associated with increasing greenhouse gases in the atmosphere, which trap the heat radiation from the Earth, much like a blanket traps the heat from a human body. The concentrations of greenhouse gases, such as carbon dioxide and methane, are rising due to human activities, and this is increasing the thickness of the blanket and warming-up the ‘skin’ of the planet.
How do we know this is happening?
The records of temperature measured at many points on the Earth’s surface clearly show a warming of about 0.8C since 1900, with most of this warming occurring since 1980.
But is there irrefutable evidence of a link between a rise in greenhouse gases and a rise in global temperatures?
Almost. The theory underlying the greenhouse effect is sound (and long-standing), and this leads to predictions that are consistent with global warming that has been measured in the last 100 years.
Some scientists argue that global temperature fluctuations are simply part of a wider natural cycle.
It is true that the Earth’s climate has varied massively in the past, even before humans were around to meddle with it. So there is always the possibility that any climate trend is a result of what you mention. This is why the statements of the Inter-governmental Panel on Climate Change (IPCC) appear to be cautious. In its last report the IPCC stated that the recent global warming is consistent with the greenhouse effect due to human activities, with a 90 per cent probability. In other words, there is only a one-in-10 chance that global warming is a natural phenomenon. The IPCC is by its very nature cautious, and many climate scientists (including myself) would put the probability that we are seeing human impacts on the climate system even higher than 90 per cent (nearer to 99 per cent in my case).
Computer predictions have been wrong before.
The underlying science of the greenhouse effect is sound and dates back to the early 19th century, long before computer models. Based on this science, Swedish scientist Svante Arrhenius estimated how much global warming would occur due to an increase in carbon dioxide, and the value he came up with is very similar to the values global climate models produce today. What the latest models can provide is guidance on where, when and how climate change will occur. These projections will always be uncertain, but they justify action to tackle global warming.
Is it true that naturally occurring water vapour poses a problem?
Water vapour is also a greenhouse gas, and the amount of water vapour that the air can hold increases strongly with the temperature of the air. So here we have a ‘feedback loop’ on warming: the temperature of the air increases because of increasing carbon dioxide, this increases the quantity of water in the air, which increases the greenhouse effect due to water vapour, and this also increases the warming. This water vapour feedback almost doubles the climate change due to a given increase in carbon dioxide. However, water vapour is not under our direct control, so it’s an amplifier of global warming rather than the cause of it.
If temperatures increase at their current rate, what will the effect on the environment be?
The amount of climate change we experience over the next century is very dependent on the extent to which humankind controls the growth of carbon dioxide emissions, and is also uncertain because we don’t know exactly how the various climate feedback processes will operate. This translates into a global warming by 2100 of somewhere between 2 and 6C. The upper end of this range is equivalent to the warming between the last ice age and now, but 100 times faster, so many believe this scenario would be impossible to adapt to. We need to be towards the very lower end of this range to avoid a warming that would ultimately melt the Greenland ice-sheet, leading to a 7m sea-level rise. This itself requires that global emissions (which are currently increasing at more than three per cent per year) need to start coming down within a decade, and be at less than half of current levels by 2050. For the developed world, this requires a cut of 80 per cent.
What can and should be done to mitigate global warming?
We need to develop energy sources that do not result in emissions of greenhouse gases. These include carbon capture and storage schemes (which remove and bury the CO2 from the flue-gases of power stations); renewable energy sources (such as solar, wind and wave); nuclear energy; and the use of hydrogen as a fuel for powering vehicles. Some of these energy supply technologies are available now, but we need to implement them to a level that avoids dangerous climate change. In the meantime, I think policy should also focus on reducing the carbon dioxide emissions from deforestation, which accounts for about 20 per cent of global carbon dioxide emissions and has numerous other detrimental impacts on the world at large.