Every April since I first read it, in 2004, I take down and re-read some portions of my copy of The Discovery of Global Warming (Harvard), by Spence Weart. (The author revised and expanded his book in 2008.) I never fail to be moved by the details of the story: not so much his identification of various major players among the scientists – Arrhenius, Milankovitch, Keeling, Bryson, Bolin – but by the account of the countless ways in which the hypothesis that greenhouse gas emissions might lead to climate change was broached, investigated, turned back on itself (more than once), debated and, eventually, confirmed.

In the Sixties, Weart trained as an astrophysicist. After teaching for three years at Caltech, he re-tooled as a historian of science at the University of California at Berkeley. Retired since 2009, he was for thirty-five years director of the Center for the History of Physics of the American Institute of Physics, in College Park, Maryland.

This year, too, I looked at the hypertext site with which Weart supports his much shorter book, updating it annually in February, incorporating all matter of new material. It includes recent scientific findings, policy developments, material from other histories that are beginning to appear. The enormous amount of material is daunting. Several dozen new references were added this year, ranging from 1956 to 2021, bringing the total to more than 3,000 references in all. Then again, all that is also reassuring, exemplifying in one place the warp and woof of discussion taking pace among scientists, of all sorts, that produces the current consensus on all manner of questions, whatever it happens to be. Check out the essay on rapid climate change, for example.

Mainly I was struck by the entirely rewritten Conclusions-Personal Note, reflecting what he describes as “the widely-shared understanding that we have reached the crisis years.”

 Global warming is upon us. It is too late to avoid damage — the annual cost is already many billions of dollars and countless human lives, with worse to come. It is not too late to avoid catastrophe. But we have delayed so long that it will take a great effort, comparable to the effort of fighting a world war— only without the cost in lives and treasure. On the contrary, reducing greenhouse gas pollution will bring gains in prosperity and health. At present the world actually subsidizes fossil fuel and other emissions, costing taxpayers some half a trillion dollars a year in direct payments and perhaps five trillion in indirect expenses. Ending these payments would more than cover the cost of protecting our civilization.

Plenty else is going on in climate policy. President Biden is hosting a virtual Leaders Summit on Climate on Thursday (Earth Day) and Friday. Nobel laureate William Nordhaus pushes next month The Spirit of Green: The Economics of Collisions and Contagions in a Crowded World (Princeton), reinforcing Weart’s conviction that it actually costs GDP not to impose a carbon tax on polluters.  Public Broadcasting will roll out later this month a three-part series in which the BBC follows around climate activist Greta Thunberg in “A Year to Change the World.” And Stewart Brand, who in 1967 published the first Whole Earth Catalog, with its cover photo of Earth seen from space, is the subject of a new documentary, We Are as Gods, about to enter distribution. There is other turmoil as well. But if you are looking for a way to observe Earth Day, reading Spencer Weart’s summing-up is an economical solution.