Climate Change - What share of warming is caused by human activity, and what is the risk?

Scientists at the Geological Society of London say -
Here is the key part of the statement from GeolSoc:
Before the current warming trend began, temperatures in the Holocene (the last 11,000 years) were declining. 
Astronomical calculations indicate that this period of low insolation and associated cool conditions should continue for about another 1,000 years. 
Nevertheless, after 1900 the overall decline in temperature sharply reversed. 
There is now greater confidence than in 2010 that the only plausible explanation for the rate and extent of temperature increase since 1900 is the exponential rise in CO2 and other greenhouse gases in the atmosphere since the Industrial Revolution.


 How can has this be proved?
The key is found in looking at the evidence of the Ice Age.
The last major glacial stage of the "Ice Age" ended thousands of years ago.

Since then, temperatures have changed, but the recent rise is very different.
The graph shows that as the ice melted, temperatures continued to rise for a few thousand years.
Around 10,000 years ago the global temperature became mainly stable.
Then around 5,000 years ago the Earth began to slowly cool, and that continued until modern times, with some variations.
So left to itself the Earth would be cooling, which would have led to a new glacial stage after thousands of years.


Glacial stages are linked to regular patterns in the movements of the Earth.

The ice would start to return in around 1,500 years.

 So the current rise in temperatures is not part of a natural cycle.



The natural cooling trend has been reversed.

This temperature rise is the effect of extra carbon dioxide.

The carbon dioxide has been produced mainly by burning fossil fuels.

(In the graph, Law Dome is a site in Antarctica where ice cores have been drilled to discover the ancient CO2 values - Mauna Loa is a site in Hawaii where the CO2 value has been measured in the air since the 1950s)

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