The record rainfall and storm surges that have brought flooding across the UK are a clear sign that we are already experiencing the impacts of climate change.
Many commentators have suggested that we are suffering from unprecedented extreme weather. There are powerful grounds for arguing that this is part of a trend.
Four of the five wettest years recorded in the UK have occurred from the year 2000 onwards. Over that same period, we have also had the seven warmest years.
That is not a coincidence. There is an increasing body of evidence that extreme daily rainfall rates are becoming more intense, in line with what is expected from fundamental physics, as the Met Office pointed out earlier this week.
A warmer atmosphere holds more water. Add to this the increase in sea level, particularly along the English Channel, which is making storm surges bigger, and it is clear why the risk of flooding in the UK is rising.
But it is not just here that the impacts of climate change have been felt through extreme weather events over the past few months. Australia has just had its hottest year on record, during which it suffered record-breaking heatwaves and severe bushfires in many parts of the country. And there has been more extreme heat over the past few weeks.
Argentina had one of its worst heatwaves in late December, while parts of Brazil were struck by floods and landslides following record rainfall.
And very warm surface waters in the north-west Pacific during November fuelled Typhoon Haiyan, the strongest tropical cyclone to make landfall anywhere in the world, which killed more than 5,700 people in the Philippines.
This is a pattern of global change that it would be very unwise to ignore.
The Intergovernmental Panel on Climate Change last September pointed to a changing pattern of extreme weather since 1950, with more heatwaves and downpours in many parts of the world, as the Earth has warmed by about 0.7C.
The IPCC has concluded from all of the available scientific evidence that it is 95% likely that most of the rise in global average temperature since the middle of the 20th century is due to emissions of greenhouse gases, deforestation and other human activities.
The upward trend in temperature is undeniable, despite the effects of natural variability in the climate which causes the rate of warming to temporarily accelerate or slow for short periods, as we have seen over the past 15 years.
If we do not cut emissions, we face even more devastating consequences, as unchecked they could raise global average temperature to 4C or more above pre-industrial levels by the end of the century.
This would be far above the threshold warming of 2C that countries have already agreed that it would be dangerous to breach. The average temperature has not been 2C above pre-industrial levels for about 115,000 years, when the ice-caps were smaller and global sea level was at least five metres higher than today.
The shift to such a world could cause mass migrations of hundreds of millions of people away from the worst-affected areas. That would lead to conflict and war, not peace and prosperity.
In fact, the risks are even bigger than I realised when I was working on the review of the economics of climate change for the UK government in 2006. Since then, annual greenhouse gas emissions have increased steeply and some of the impacts, such as the decline of Arctic sea ice, have started to happen much more quickly.
We also under-estimated the potential importance of strong feedbacks, such as the thawing of the permafrost to release methane, a powerful greenhouse gas, as well as tipping points beyond which some changes in the climate may become effectively irreversible.
What we have experienced so far is surely small relative to what could happen in the future. We should remember that the last time global temperature was 5C different from today, the Earth was gripped by an ice age.
So the risks are immense and can only be sensibly managed by reducing greenhouse gas emissions, which will require a new low-carbon industrial revolution.
History teaches us how quickly industrial transformations can occur through waves of technological development, such as the introduction of electricity, based on innovation and discovery.
We are already seeing low-carbon technologies being deployed across the world, but further progress will require investment and facing up to the real prices of energy, including the very damaging emissions from fossil fuels.
Unfortunately, the current pace of progress is not nearly rapid enough, with many rich industrialised countries being slow to make the transition to cleaner and more efficient forms of economic growth.
The lack of vision and political will from the leaders of many developed countries is not just harming their long-term competitiveness, but is also endangering efforts to create international co-operation and reach a new agreement that should be signed in Paris in December 2015.
Delay is dangerous. Inaction could be justified only if we could have great confidence that the risks posed by climate change are small. But that is not what 200 years of climate science is telling us. The risks are huge.
Fortunately poorer countries, such as China, are showing leadership and beginning to demonstrate to the world how to invest in low-carbon growth.
The UK must continue to set an example to other countries. The 2008 Climate Change Act, which commits the UK to cut its emissions by at least 80% by 2050, is regarded around the world as a model for how politicians can create the kind of clear policy signal to the private sector which could generate billions of pounds of investment. Weakening the Act would be a great mistake and would undermine a strong commitment made by all of the main political parties.
Squabbling and inconsistent messages from ministers, as well as uncertainty about the policies of possible future governments, are already eroding the confidence of businesses. Government-induced policy risk has become a serious deterrent to private investment.
Instead, the UK should work with the rest of the European Union to create a unified and much better functioning energy market and power grid structure. This would also increase energy security, lower costs and reduce emissions. What better way is there to bring Europe together?
The government will also have to ensure the country becomes more resilient to those impacts of climate change that cannot now be avoided, including by investing greater sums in flood defences.
It should resist calls from some politicians and parts of media to fund adaptation to climate change by cutting overseas aid. It would be deeply immoral to penalise the 1.2 billion people around the world who live in extreme poverty.
In fact, the UK should be increasing aid to poor countries to help them develop economically in a climate that is becoming more hostile largely because of past emissions by rich countries.
A much more sensible way to raise money would be to implement a strong price on greenhouse gas pollution across the economy, which would also help to reduce emissions. It is essential that the government seizes this opportunity to foster the wave of low-carbon technological development and innovation that will drive economic growth and avoid the enormous risks of unmanaged climate change. More
• Nicholas Stern is chair of the Grantham Research Institute on Climate Change and the Environment at the LSE and president of the British Academy.
