Geert Jan is dead. He, never one to mince words, would have stated that fact in exactly this way. For those of us who survive him, this is a hard reality that will take a long time to come to terms with. With his passing, climate scientists around the world have lost a mentor, a teacher, a colleague, a friend and an incredible role model.

Credit: Robert Vautard

Despite appearances and repeatedly calling himself one, Geert Jan was, in all his work, his attitude to life and his friendship and collegiality, the opposite of the stereotypical old white man. He was everything a scientist should be — formidably clever with a superhuman capacity to work — but what made him so important was his insistence not only on getting the science right but also on doing the right thing. For him, this meant making science not an endeavour for the privileged alone but for everyone who wanted to understand the natural world, in particular the atmosphere around us, how the weather is changing in a warming world and what the limits of the data and scientific methods are. To achieve this, he created a website, the Climate Explorer (, making climate models and data available to anyone with even the poorest internet connection. Ever accessible, the Climate Explorer had a help function, creating emails that were sent directly to him and that he always answered, even when his time got more and more precious during his illness. He was similarly generous with his time with colleagues and journalists seeking help and advice from across the world. It was this that made Geert Jan highly influential.

Geert Jan had wanted to answer questions that actually matter, although not on his own but within a community of likeminded scientists. And that is what he created, through generosity and first class science. He was no saint; he knew when he was right, and he never let anyone doubt that, to the detriment of some big egos in our community. Sometimes, this was also to his own detriment as, not infrequently, those with other opinions were reviewers or editors. It is not completely without irony that this obituary is published in a Nature journal.

Later in his career, his influence became more visible as one half of the World Weather Attribution leadership team (; the other half is myself. When we started, I was a postdoc who had authored a single paper, which Geert Jan helped to write, of course. Despite this, we were true partners from day one, having a similar background, a joint vision and complementary skills. He had the scientific experience, statistical skill and absolute drive to get the numbers right; I had the words to explain what the numbers say in simple terms. When we started, colleagues laughed at us, saying the idea would never fly. Now, six years later, we are two of Time magazine’s 100 most influential people of 2021 (McKibben, B. Time (14 September 2021); World Weather Attribution is flying, thanks in large degree to the emphasis on openness and transparency in the way we do the science, which had been Geert Jan’s approach all along. Geert Jan’s determination to say what we can say (but no more) changed how we talk about weather, and this has resulted in a decrease in public scepticism about climate change. I wish he would have lived to see us acting on it too.

What unites Geert Jan’s two big projects, Climate Explorer and his attribution work, and is the core of his scientific legacy, is bringing observations and climate models together as two different lines of evidence to answer the same questions. Nowadays, this may seem to be the obvious thing to do, but it had not been done previously. For the longest time, the communities making and interpreting observations and those building and analysing climate models were strictly separated. In the Climate Explorer, you can look at both types of data in exactly the same way, which is also what we do in event attribution: bringing the past and future together to make sense of the present. He understood the limitations of both types of data as well as their advantages, allowing him to depart from established wisdom and push the boundaries on which questions could be answered. Geert Jan not only enabled “the attribution of extreme weather events to graduate from an eccentric and controversial idea largely confined to a single university into a credible field of science”, as his colleague and friend Myles Allen put it, but he helped the whole scientific community to recognize the importance of using all of the lines of evidence together.

By walking the walk, never preaching and just doing what was right, Geert Jan pulled a whole community out of its comfort zone and helped us all to become much more relevant. Geert Jan’s scientific legacy lives on in all of us who knew or read him. His work lives on, but many of us have lost a friend. Personally, it is hard to come to terms with knowing that he will no longer severely violate the limits of my own comfort zone by dragging me on sailing boats, exposing me to Gamelan music or discussing ancient sci-fi literature.

Geert Jan passed on 12 October 2021, having fought with Kahler’s disease, a malignant cancer of the bone marrow, since 2013. My thoughts are with his wife and his three sons, all four of whom Geert Jan was incredibly proud.