Special Report-The methodology behind the Reuters Hot List

Maurice Tamman
·5-min read

By Maurice Tamman

(Reuters) - This series tells the stories of the scientists who are having the biggest impact on the climate-change debate – their lives, their work and their influence on other scientists, the public, activists and political leaders.

To identify the world's most influential climate scholars, we created the Hot List, which is a combination of three rankings. Those rankings are based on how many research papers scientists have published on topics related to climate change; how often those papers are cited by other scientists in similar fields of study, such as biology, chemistry or physics; and how often those papers are referenced in the lay press, social media, policy papers and other outlets.

The data is provided through Dimensions, the academic research portal of the British-based technology company Digital Science. Its database contains hundreds of thousands of papers related to climate science published by many thousands of scholars, the vast majority published since 1988.

The list combines three rankings:

For the first ranking, we selected researchers based on the number of papers published under their names through December 2020, as indexed in the Dimensions system. We screened for climate-related work by examining the papers' titles or abstracts – brief descriptions of the research – for phrases closely connected to climate change, such as "climate change" itself, global warming, greenhouse gases and other related terms. These are papers that explicitly focus on climate change rather than mention it in passing. To be included in our count, a paper had to be cited by at least one other scientist at least once.

The first ranking is based on how many papers meet that criteria for each scientist. A rank of one was given to the scientist with the most papers, and 1,000 to the scholar with the fewest.

The second ranking is based on what Dimensions describes as a "Field Citation Ratio." For each paper, a ratio is calculated "by dividing the number of citations a paper has received by the average number received by documents published in the same year and in the same Fields of Research category," according to Dimensions. This ranking is meant to measure the influence of scientists' work among their peers.

For example, atmospheric sciences, a subset of earth sciences, is a field of research, as is zoology, which belongs to the biological group of sciences. A zoology-related paper with a ratio of 1.0 means it was cited at the average rate compared to other zoology papers; a paper with a score of 2.0 means it was cited at twice the rate of the average zoology paper. Climate change is a multidisciplinary science, and this approach accounts for differing citation rates in differing fields.

For the Hot List, we calculated an average citation ratio for each scientist's climate-change papers, then we ranked the ratios of all the scholars on our list. A rank of one was assigned to the scholar with the highest average ratio, and 1,000 to the researcher with the lowest.

The third ranking is based on Digital Science's Altmetric Attention Score, a measure of a research paper's public reach. Most papers receive a score based on references in a variety of publications, including the mainstream media, Wikipedia, public policy papers and social media sites such as Twitter and Facebook. The ranking is meant to measure the influence of scientists' work in the lay world.

For the Hot List, we assigned a median Altmetric score to each scientist's papers and then ranked those scores, with a rank of one going to the highest score and 1,000 to the lowest.

The final score for each scientist is based on the sum of each ranking – the lower the score, the greater the scholar's overall influence, and thus the higher he or she ranks on the Hot List.

For example, Keywan Riahi, the head of Austria's International Institute for Applied Systems Analysis, is the highest-ranking scientist on the Hot List. He ranks 47th for papers published, 10th for his Field Citation Ratio and 30th for his Altmetric Attention Score, for a score of 87.

Riahi, who studies energy systems, said his ranking is probably the result of IIASA's openness to sharing data and models with other scientists. "That creates long-standing collaborations, and, of course, when we innovate, we pass innovation on, and all that's important for the scientific network," he said.

Some notes of caution. First, the Hot List doesn't claim to be a rank of the "best" or "most important" climate scientists in the world. It's a measure of influence.

Second, the Hot List has some limitations inherent in our methodology. For instance, our analysis targeted the titles and abstracts of papers, not the full texts, so we may have missed some studies that do touch on climate change. The Altmetric score can be skewed upward if one or a few of a scientist's papers have particularly high scores and their remaining papers have comparatively low scores.

Also, the Hot List favors the prolific. The first of our three metrics ranks scientists based on the number of papers published. The other two metrics – for citation ratios and public reach – are designed to compensate for this possible bias, but they might not fully do so.

(Reporting by Maurice Tamman; editing by Kari Howard)