We're already past the point of dangerous [manmade] interference with the climate system — not approaching it — past it," said Jeffrey Sachs, an economist who directs Columbia's Earth Institute.
NASA climatologist James Hansen said that to date, we've only seen about half the warming that is likely to occur due to the greenhouse gases that have already been emitted, yet we're already feeling major impacts. The climate system responds slowly to the added heat from greenhouse gases, with the oceans taking much longer to warm compared to land surfaces. This means that even if all emissions were to be stopped today, warming would continue for several decades.
"That inertia is not our friend, because it can cause the climate system to pass tipping points," Hansen said.