Mark Lynas: What an increase of 3.5C really means to the planet

Sea levels are creeping higher, polar bears are history and tropical storms of undreamt-of ferocity batter the world's coastlines.

Welcome to the world bequeathed to us by negotiators at Copenhagen last year, whose timid proposals for cutting back on carbon emissions will do little to turn the tide of global warming. A world 3.5C hotter will be well outside the safety zone, currently estimated as between 1.5 and C by scientists.

Once global temperatures pass 3C, several crucial tipping points in the Earth's climate system are likely to have been crossed. Firstly, the ice cap over the North Pole will have disappeared entirely during the summer months, changing the planet's energy balance and weather patterns. Secondly, melting permafrost in Siberia and other high-latitude areas will be releasing millions of tonnes of the extra-powerful greenhouse gas methane, and there will be nothing we can do to stop it. And lastly, the world's most important and biodiverse tropical forest, the Amazon region, will be burning up and transforming into desert.

Life for humans will be getting increasingly hot and sticky. Saharan-type temperatures, well over 50C, will be striking regularly in summertime continental interiors, from the southern United States to the south Asian subcontinent to the Middle East. Around the Mediterranean, forests will be tinder-dry and devastating wildfires an annual occurrence – Australia and California can expect much of the same.

Deadly heatwaves, such as that which struck Europe in 2003 and Moscow in 2010, will be a normal summer. Global warming means more energy is available to drive the hydrological cycle, and in addition a warmer atmosphere holds more water vapour. These two factors are already behind an intensification in heavy rainfall events from England's recent Cumbrian floods to the flooding disaster in Pakistan.

In the 3.5C world, the oscillation between drought and flood will be even more profound: in Africa and India the rains may fail one year, only to be replaced by topsoil-stripping monsoons the following season. Global food supplies will be stretched by the unpredictability of weather extremes, and the return of dustbowl conditions to the world's major breadbaskets such as the US corn belt.

Sea levels by the end of the century will be heading towards two metres above today's high tide mark, dooming low-lying nations such as the Maldives and endangering coastal megacities like Shanghai and Bangkok – not to mention New York and London. Most of the additional water will be sluicing off the Greenland ice-cap – now in irreversible melt – and the rapidly-diminishing remains of Himalayan and Andean glaciers. The warmer oceans will be stagnant in many areas, spreading low-oxygen "dead zones" over vast areas and ruining fisheries the world over.

Just as distressing will be a devastating extinction event in what remains of the natural world: perhaps as much as half of all the world's plants and animals may be heading for extinction. Humans may survive, but our warmer world will be increasingly lonely.