Earth's past warmth is no get-out clause

CLIMATE scientists have long held that the past 2000 years were almost uniformly cool. Now it seems they were wrong. The 1st century AD looks to have been as warm as today, and the world gradually cooled from then on, right up until the industrial era (see "Tree rings suggest Roman world was warmer than thought").

Climate change denialists, who have never accepted that we are in unusually warm times, will say "told you so". They may also claim that scientists are trying to have it both ways - whatever past temperatures were, they are still evidence of global warming today.

Yet once again, they would be misrepresenting the evidence. The new finding confirms the primacy of human-made carbon dioxide emissions in the warming of the past century. A long-term downward trend in temperature makes it even less likely that recent warming could be due to normal variability.

Human activity seemingly snuffed out the natural cooling, caused by orbital wobbles, that was carrying us into the next ice age. We may or may not regard that as good news. But we cannot deny that our hand is still on the thermostat, and is cranking it in the wrong direction.

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