Philip James de Loutherbourg (1801) Madeley Wood Furnaces, Coalbrookdale, Wikicommons
In discussions like those held at the UN Climate Change conference in Paris in 2015, we talk about limiting the increase in global average temperatures to 2C. But 2C warmer than what?
We are aiming to keep global average temperature to less than 2C higher than the pre industrial global average temperature. Or, more simply the global average temperature at the point just before the industrial revolution began in earnest. That is the baseline.
But what if we're not going back far enough in time? What if the global average temperature in the late 1800's, which is the period current baselines come from, were already an increase on a true pre-industrial average?
Evidence strongly suggests that greenhouse gases were already changing the climate in the late 1800's; global average temperatures were already rising compared to say the early 1700s. So the baseline temperature we are using was probably already 0.2C warmer than the true 'pre industrial' global average.
What this means for our present day emissions and warming targets is explored by Michael Mann and others in a new paper published by Nature.