In an accompanying News & Views article in Nature, Prof Matthew Kirby, a professor of palaeoclimatology at California State University who wasn't involved in the study, tackles this exact question: (carbonbrief.org)
Haywood, Alan Alan Haywood is Reader in Palaeoclimatology at the School of Earth and Environment, University of Leeds, UK, and a recent recipient of the Philip Leverhulme Prize. (copenhagendiagnosis.com)