Avoiding the Unadaptable: A 4°C World

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Avoiding the Unadaptable: A 4°C World
Avoiding the Unadaptable: A 4°C World
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With increasing concern that global emission reductions are too little and too late to limit temperature rise to 2°C or less, attention is turning to the implications of much more severe climate change.

The World Bank recently released a report – Turn Down the Heat: Why a 4°C Warmer World Must be Avoided – that directly addresses those implications. Written by scientists from the Potsdam Institute for Climate Impact Research, the report pulls together the latest research from experts around the world to profile the impacts and risks associated with a 4°C temperature rise within this century. It is a sobering read.

Where are we now?

Before we dive into the 4°C world, though, let’s take stock of where we are at the beginning of 2013. The atmospheric carbon dioxide concentration is 391 parts per million (ppm), the highest for the last 15 million years. The global average temperature has risen by 0.8°C compared to the pre-industrial level. The sea level has risen 15-20 cm over the past century, and is now rising at an average of about 3 mm per year.

Extreme climatic events are also changing, most notably heat waves. Since 1950 there has been a 10-fold increase in the land areas affected by extreme heat; the chance that such an increase would occur naturally, without any influence of human-induced climate change, is only 1-in-500. In some regions, extreme droughts and flooding may also have been exacerbated by climate change.
Bleaching on the Great Barrier Reef

Coral reefs are vulnerable to increasing ocean temperatures and ocean acidification.

What would a 4°C world by the end of this century look like?

To put such a remarkable climatic shift in perspective, it is useful to examine the geological past. A 4°C temperature rise would approach the total temperature difference between an ice age and a warm period. While the Earth took at least 5,000 years to transition out of the last ice age, allowing ecosystems time to adapt, the projected 4°C world would be upon us in only 100 years!

A 4°C average temperature rise means even hotter conditions over the land, up to 6°C over widespread areas. Today’s most extreme weather would become the “new normal”, with large increases in the risks of heat-related deaths, intense bushfires and crop harvest losses.

Ocean acidification would lead to the extinction of many of the world’s coral reefs. A 4°C world corresponds to a carbon dioxide concentration of around 800 ppm, and a 150% increase in ocean acidity compared to pre-industrial. Coral growth virtually stops at 450 ppm CO2 and hard coral structures start to dissolve at 550 ppm.

Sea level is set to rise by a further 0.5 to 1 m by 2100, but a 4°C temperature rise this century would push the odds towards the upper end of that range. Sea-level rise will continue for many centuries after 2100, given the thermal inertia in the oceans and large polar ice sheets. Evidence from climate shifts in Earth’s past shows that a 1.5°C global temperature rise would mean an eventual 5-10 m sea level rise, while a sustained 4°C world would commit us to significantly larger rises. The most vulnerable areas to even modest levels of sea-level rise are many of the small island states and coastal cities in India, Bangladesh, Indonesia, the Philippines and Vietnam.

The implications for human support systems – food, water and ecosystem health – are severe. There is some evidence that wheat and maize production may have been reduced since the 1980s compared to an environment without climate change. Many areas of the world are projected to become significantly drier in a 4°C world; these include southern Europe, much of Africa, parts of North and South America, and southwest and southeast Australia. Ironically, the risk of both extended and severe droughts and severe flooding is likely to increase. The rapidly changing climate that a 4°C world implies will lead to accelerating disruption to ecosystems and increase the probability of a mass extinction event this century.

Many abrupt, irreversible or surprising changes in the global environment are much more likely to occur with a 4°C rise this century compared to 2°C or less. Such changes include large-scale population displacements as rainfall patterns change; the triggering of “tipping elements” such as destabilization of the Greenland and West Antarctic ice sheets and loss of large amounts of methane, a strong greenhouse gas, from melting permafrost; and complex, cascading impacts such as the 2007-08 food crisis.
Wheat crop

The implications for food production are severe as large areas of the world are projected to become drier.

Where are we heading?

The report concluded that “…there is a strong possibility that humanity cannot adapt to a 4°C world.” Yet, current policies, trends and emission reduction commitments have us on track for just such a world.

Although the situation appears bleak, and there is a growing sense of panic in those who really understand what a 4°C world might be like, the report also gives some hope, “But with action, a 4°C world can be avoided and we can likely hold warming below 2°C.”

The time for that action, however, is rapidly running out.

Author: Will Steffen
Tags: 4°C world, Climate Commission, World Bank
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