Reducing greenhouse gas emissions soon and fast enough to prevent a global average temperature rise of more than 2 °C above the pre-industrial level commonly regarded as the threshold for ‘dangerous climate change’ was always going to be tough. As we approach the end of the first decade of the twenty-first century and emissions continue to increase, keeping below 2 °C with any reasonable probability will be possible only with urgent and stringent mitigation measures. In practice, this will necessitate almost immediate emissions reductions by rich nations, followed soon after by reductions from developing nations.

As the challenge of keeping below 2 °C increases, the likelihood of reaching higher temperatures becomes correspondingly larger. At the worst end of the scale, with continued intensive fossil fuel use, temperatures could rise 4 °C by the 2070s, or even as early as 2060 if there are strong positive feedbacks in the carbon cycle. The situation is bleaker still once political inertia is considered. Moderate-emissions scenarios, including those arising from weak climate agreements, still result in a significant probability of exceeding 4 °C by the end of the century or early in the next century. As nations delay on agreeing a global climate treaty, it seems essential to explore the terra quasi-incognita of a world in which the average temperature is 4 °C above the pre-industrial level, and to understand the implications for nature and society.

Brave new world

A world where the average temperature was 4 °C higher than in pre-industrial times would be very different from the one we now inhabit, and even from one with 2 °C of warming. Studies suggest that 2-4 °C of warming would trigger the permanent break-up of the Greenland ice sheet, causing sea level to rise by up to seven metres in the long term. With warming of 3 °C, the Arctic Ocean would most likely be ice-free in summer. At 4 °C, most reef-building corals would be unable to adapt to changes in ocean temperature and acidification, in which case tropical coral reefs would die out or become far less diverse. While thresholds or tipping points in other systems are less well known, the risk of major shifts in ecosystems such as tropical forests increases as global temperature rises from 2 to 4 °C.

A 4 °C, the world would probably be warmer than any time in the last 800,000 years10 and certainly the last 18,000 years, the period in which modern humans evolved. Moreover, the rate of climate change would be as fast as or faster than any previously experienced. Because land areas warm faster than the ocean and higher latitudes more than lower latitudes, temperature increases would exceed 4 °C in many regions. Approximately 13 per cent of land including the Amazon, the Sahara-Sahel-Arabia region, India and northern Australia could experience average temperatures for which there are no spatial analogues in today’s climate; in other words, the temperature in these regions would be higher than the average at any place on Earth today. Correspondingly, present-day climates in the tropics and subtropics would shift short distances to higher elevations or in some cases several thousands of kilometres polewards.