The latest report from the world’s climate scientists has made clear the size of the challenge if the world is to stay below the global warming limit hoped for in the Paris climate agreement. Unfortunately, with current trends we are likely to cross this threshold within the next two decades because we are already two-thirds of the way there.

But how do we know what is driving these climate trends? It comes down to the same kind of detective work that typifies a crime scene investigation, only here we are dealing with a case that encompasses the whole world. Let me give you my view, which does not necessarily represent the position of NASA or the federal government.

For the past 100 years we have documented good, independently confirmed observations of change at the surface of the planet, and for the past 40 years satellites and comprehensive measuring efforts have provided a much fuller view of changes throughout the earth system. These observations show clearly that among other things, the surface of the planet has warmed, the upper atmosphere has cooled, the oceans are gaining an enormous amount of heat, sea level is rising, Arctic ice has greatly receded and glaciers around the world are in retreat.

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Scientists have no shortage of suspects for the causes of climate change. Over the 4.5-billion-year history of the planet, almost anything that might have happened has happened, sometimes many times over. The sun has brightened and sometimes dimmed, massive volcanic eruptions have occurred, the planet’s orbit has wobbled, the continents have moved, the land surface has been remade, the composition of the atmosphere and its ability to trap and reflect solar and terrestrial radiation has altered, ocean circulations have sputtered and stopped, and we have been struck, at least once, by a mass-extinction-inducing asteroid.