One big global challenge

Cooling an entire planet 

is an immense task in its own right.

But to avert catastrophe, 

any geoengineering scheme

must also meet several other requirements.

Some time this century 

we are likely to have doubled 

the concentration of CO2 in the atmosphere, 

reducing heat loss by about 3.7 watts 

per square metre, averaged across the planet.

We have other prerequisites 

for our global refrigerator. 

It needs to work without 

drastically altering regional climates, 

while also preventing sea level from rising. 

Ideally we want to stop the oceans 

becoming so acidic that coral reefs vanish, too.

STOP THE WARMING

The higher CO2 levels rise,

the harder it will be to prevent

catastrophic temperature rises.

The dramatic warming in the Arctic

will not be easy to reverse, 

even with a lower global temperature.

Ice loss from Greenland

will accelerate if we don't stop

the Arctic warming.

The vast East Antarctic ice sheet

is already losing ice.

Merely halting warming 

might not prevent the eventual collapse

of the West Antarctic ice sheet.

MAINTAIN A STABLE CLIMATE

Drastic cooling schemes

like global sunshades could reduce global rainfall

and lead to disastrous changes in regional climates.

Altering rainfall patterns could turn 

the Amazon rainforest into desert.

Disrupting the Asian monsoon

would  cause billions to starve.

HALT SEA LEVEL RISE

Only cooling the poles back down to preindustrial levels

will stop a huge rise in sea levels, yet many geoengineering

schemes will not cool the poles as much as the tropics.

It may already be too late 

to save many low-lying

coastal areas from rising seas.

STOP OCEAN ACIDIFICATION

The only way to do this is to remove 

CO2 from the oceans or atmosphere.

No one has found a feasible way

to capture enough of it.

If we do not remove CO2 from the oceans,

coral reefs will start to disappear.

http://www.newscientist.com/data/images/archive/2883/28831701.jpg