This was one of the biggest volcanic explosions in history
Daybreak and Mount Pinatubo bursts angrily into life again
It happened on June 15th 1991...
...in the north-west of the Philippines
It was so powerful...
...it produced a gas cloud that reached the stratosphere
The explosion caused a lot of damage locally...
...but the cloud itself did something extraordinary...
...it lowered the Earth’s temperature for four years
Sulphur dioxide in the cloud created particles...
...which spread around the Earth
These then reflected some of the sun’s rays into space
Scientists are looking to mimic the effects of this phenomenon...
...to counter global warming
It’s a highly controversial concept...
...known as solar geoengineering
Climate change is probably the biggest problem...
...humanity faces today
In the past 25 years...
...the global average temperature has risen by 0.5°C
The accumulation of carbon in the atmosphere is warming the Earth up...
...making more intense hurricanes, rains, higher temperatures
So far governments have focused on policies for cutting emissions...
...but they keep rising
The concentration of carbon dioxide in the atmosphere...
...has risen 15% since 1994
The Paris agreement, signed by 175 parties in 2016...
...was a sign that countries were willing to work together...
...to cut emissions
But not every world leader is on board
The Paris Climate Accord is simply the latest example of...
...Washington entering into an agreement that disadvantages...
...the United States
If emissions aren’t reduced, what next?
Theoretically solar geoengineering could cool the entire planet
It’s actually potentially incredibly simple because...
...all you need to do is inject a bunch of particles into the stratosphere...
...and there’s a number of ways you could do that
Recently a study out of Yale and Harvard...
...looked at the details of how this would be done
In that Harvard and Yale study...
...they imagined building a fleet of planes...
...up to 95 of them...
...that would make 60,000 flights a year
The fleet would spread hundreds of thousands of tonnes...
...of sulphur dioxide into the stratosphere every year
After 15 years...
...they reckon the world would cool by 0.3°C
But there are reasons not to do this
One of the biggest concerns...
...is that it could make governments complacent
The big worry...
...is that it will be exploited to argue against emissions cuts
Geoengineering is a supplement to cutting emissions...
...not a substitute for cutting emissions
It would also be politically messy
If the plan is to put a thermostat on the Earth...
...deciding who has their hand on the dial...
...won’t be easy
For it to have a global effect...
...you’re going to need all governments on board...
...and that is going to be extremely difficult
If a country went alone and decided to do it on its own...
...then obviously because the impacts are going to be regional...
...or global, you could create a lot of international tension 
A Swiss proposal to study geoengineering...
...and how it should be regulated...
...was recently put forward at the UN Environment Assembly
But America and Saudi Arabia opposed the motion...
...possibly because they don’t want...
...international regulation of geoengineering
As things stand...
...not enough is known about how it could impact the climate...
...or the chemistry of the atmosphere
There could be unexpected consequences
You can affect rain patterns...
...and you might cause a region that is...
...already suffering from drought...
...to have even less water
A lot more basic research both into...
...how we would do it but also...
...what the consequences of it would be...
...if we did it in one way or in another way is needed
But of course if we don’t do it...
...there’s deep uncertainty about how much climate change we get...
...for a given amount of CO₂ in the air
Both of those are risky, both are uncertain, the question is...
...which has higher risk?
The politics of solar geoengineering are so complex...
...that it might never happen
That said, as the world continues to warm...
...the case for exploring radical measures...
...grows stronger
