Well, at least that clears up the mystery. Over the past year I’ve been fretting over an intractable contradiction. The government has promised spectacular cuts in greenhouse gas emissions. It is also pushing through new roads and runways, approving coal-burning power stations, bailing out car manufacturers and ditching regulations for low-carbon homes. How can these policies be reconciled?

We will find out tomorrow, when it publishes a series of papers on carbon reduction. According to one person who has read the drafts, the new policies will include buying up to 50% of the reduction from abroad. If this is true, it means that the UK will not cut its greenhouse gases by 80% by 2050, as the government promised. It means it will cut them by 40%. Offsetting half our emissions (which means paying other countries to cut them on our behalf) makes a mockery of the government’s climate change programme.

The figure might have changed between the draft and final documents, but let’s take it at face value for the moment, to see what happens when rich nations offload their obligations. What I am about to explain is the simple mathematical reason why any large-scale programme of offsets is unjust, contradictory and ultimately impossible.

Last week the G8 summit adopted the UK’s two key targets : it proposed that developed countries should reduce their greenhouse gases by 80% by 2050 to prevent more than two degrees of global warming. This meant that it also adopted the UK’s key contradiction, as there is no connection between these two aims. An 80% cut is very unlikely to prevent two degrees of warming; in fact it’s not even the right measure, as I’ll explain later on. But let’s work out what happens if the other rich nations adopt both the UK’s targets and its draft approach to carbon offsets.

Please bear with me on this: the point is an important one. There are some figures involved, but I’ll use only the most basic arithmetic, which anyone with a calculator can reproduce.

The G8 didn’t explain what it meant by “developed countries”, but I’ll assume it was referring to the nations listed in Annex 1 of the Kyoto protocol: those that have promised to limit their greenhouse gases by 2012. (If it meant the OECD nations, the results are very similar.) To keep this simple and consistent, I’ll consider just the carbon emissions from burning fossil fuels, as listed by US Energy Information Administration. It doesn’t publish figures for Monaco and Lichtenstein, but we can forgive that. The 38 remaining Annex 1 countries produce 15bn tonnes of CO2, or 51% of global emissions. Were they to do as the UK proposes, cutting this total by 80% and offsetting half of it, they would have to buy reductions equal to 20% of the world’s total carbon production. This means that other countries would need to cut 42% of their emissions just to absorb our carbon offsets.

But the G8 has also adopted another of the UK’s targets: a global cut of 50% by 2050. Fifty per cent of world production is 14.6bn tonnes. If the Annex 1 countries reduce their emissions by 80% (including offsets), they will trim global output by 12bn tonnes. The other countries must therefore find further cuts of 2.6bn tonnes. Added to the offsets they’ve sold, this means that their total obligation is 8.6bn tonnes, or 60% of their current emissions.

So here’s the outcome. The rich nations, if they follow the UK’s presumed lead, will cut their carbon pollution by 40%. The poorer nations will cut their carbon pollution by 60%.

If global justice means anything, the rich countries must make deeper cuts than the poor. We have the most to cut and can best afford to forgo opportunities for development. If nations like the UK cannot make deep reductions, no one can. We could, as I showed in my book Heat, reduce emissions by 90% without seriously damaging our quality of life. But this carries a political price. Business must be asked to write off sunk costs, people must be asked to make minor changes in the way they live. This country appears to be doing what it has done throughout colonial and postcolonial history: dumping its political problems overseas, rather than confronting them at home.

Befuddled yet? I haven’t explained the half of it. As the G8 leaders know, a global cut of 50% offers only a faint to nonexistent chance of meeting their ultimate objective: preventing more than two degrees of warming. In its latest summary of climate science, published in 2007, the Intergovernmental Panel on Climate Change suggested that a high chance of preventing more than two degrees of warming requires a global cut of 85% by 2050. In drafting the climate change act, the UK government promised to keep matching the target to the science. It has already raised its cut from 60% to 80% by 2050. If it sticks to its promise it will have to raise it again.

Global average CO2 emissions are 4.48 tonnes per person per year. Cutting the world total by 85% means reducing this to 0.67 tonnes. Average per capita output in the 38 Annex 1 countries is 10 tonnes; to hit this target they must cut their emissions by 93.3% by 2050. If the rich persist in offsetting 50% of this cut, the poorer countries would have to reduce their emissions by 7bn tonnes to absorb our offsets. To meet a global average of 0.67 tonnes, they would also need to chop their own output by a further 10.8bn tonnes. This means a total cut of 17.8bn tonnes, or 125% of their current emissions. I hope you have spotted the flaw.

In fact, even the IPCC’s proposal has been superseded. Two recent papers in Nature show that the measure that counts is not the proportion of current emissions produced on a certain date, but the total amount of greenhouse gases we release. An 85% cut by 2050 could produce completely different outcomes. If most of the cut took place at the beginning of the period, our cumulative emissions would be quite low. If, as the US Waxman- Markey bill proposes, it takes place towards the end, they would be much higher. To deliver a high chance of preventing two degrees of warming, we would need to cut global emissions by something like 10% by the end of next year and 25% by 2012. This is a challenge no government is yet prepared to accept.

Carbon offsetting makes sense if you are seeking a global cut of 5% between now and for ever. It is the cheapest and quickest way of achieving an insignificant reduction. But as soon as you seek substantial cuts, it becomes an unfair, impossible nonsense, the equivalent of pulling yourself off the ground by your whiskers. Yes, let us help poorer nations to reduce deforestation and clean up pollution. But let us not pretend that it lets us off the hook.

Planet Earth. Photograph: Blue Line Pictures/Getty Images

Barack Obama’s sense of urgency in getting Congress and the international community to act on climate change does not appear to have rubbed off on the average American, a new study published today reveals.

Even as the president pressed the G8 and the world’s major polluters to resist cynicism and the pressure of the economic recession to act against global warming, a majority of Americans remain unconvinced that humans are responsible for climate change, or that there is an urgent need to act.

About 49% of Americans believe the Earth is getting warmer because of the burning of fossil fuels and other human activity, the survey by the Pew Research Centre and the American Association for the Advancement of Science said. Some 36% attributed global warming to natural changes in the atmosphere and another 10% said there was no clear evidence that the earth was indeed undergoing climate change.

Scientists in contrast are overwhelmingly persuaded that global warming is caused by humans – some 84% blame human activity. A strong majority – some 70% – also believe it is a very serious problem. Despite that degree of consensus, some 35% of Americans continues to believe – wrongly it turns out – that climate change remains a matter of scientific controversy. Only about 47% of the public views climate change as a very serious problem, a finding that has remained stable over the years, the survey said. In other public opinion polls over the years, climate change has ranked near the bottom of the list of pressing problems.

The Pew poll, like others in the past, also found attitudes towards climate change breaking down according to political allegiance. Some 67% of Republicans either deny the existence of climate change or attribute the phenomenon to natural causes. In contrast, 64% of Democrats believe that the earth is getting warmer because of human activity.

Climate scientists have warned of wild weather in the year ahead as the start of the global “El Niño” phenomenon exacerbates the impact of global warming. As well as droughts, floods and other extreme events, the next few years are also likely to be the hottest on record, scientists say.

In the UK, a Met Office spokesman said yesterday that the El Niño event was likely to cause a hot, dry summer following a warm June, but said it could have other unpredictable effects on weather in Britain and north-west Europe. “Much depends on how much the El Niño deepens in the next few months.”

El Niño – “the child” in Spanish – was named by fishermen in Peru and Ecuador because the phenomenon arrives there at Christmas. It is part of a natural meteorological cycle that happens every 3-7 years and affects weather worldwide for a year or more. It is caused by changes in ocean temperatures, with the first sign being abnormal warming in the Pacific.

Sea surface temperatures across an area of the Pacific almost the size of Europe have been increasing for six months and will trigger worldwide weather turbulence for the next year, said a spokesman for the US National Oceanic and Atmospheric Administration (Noaa).

At this stage, both US and Australian climate scientists say this may be a medium-strength El Niño, but they have warned it could develop further.

“Temperatures in the Pacific are around 1C above average, and sub-surface temperatures up to 4C warmer than normal, ” said a spokesman for Australia’s Bureau of Meteorology.

The last major El Niño in 1998 killed more than 2,000 people and caused billions of dollars worth of damage to crops and infrastructure in Australia and Asia. It led to forest fires in south-east Asia, a collapse of fish stocks in South America and a drought threatening 700,000 people in Papua New Guinea.

Strong El Niños often have long-lasting effects. The 1991-92 event led to droughts in Africa and food shortages that left 30 million people at risk of malnutrition and set back development for a decade.

Oxfam has alerted teams globally. “This could be the hottest year in known history. Poverty and climate change is enough of a challenge: an El Niño will only make things harder,” said Steve Jennings, Oxfam’s disaster risk reduction manager.