Land areas and tropical oceans in the Northern Hemisphere have been warming since the mid-nineteenth century - earlier than the widespread climate warming that took place in the twentieth century - finds a study published in Nature this week.

Climate records of the past 2,000 years have focused on the Northern Hemisphere and have mainly used data from terrestrial records as opposed to marine ones. However, terrestrial records do not account for the importance of oceans in determining the pace of climate change and its regional implications.

Nerilie Abram and colleagues use post-AD 1500 terrestrial and marine climate records to show that the widespread climate warming observed during the twentieth century is part of a sustained trend that began in the tropical oceans and over some land areas in the Northern Hemisphere around the 1830s. In contrast to what occurs in the Northern Hemisphere, the authors’ reconstruction suggests that the development of Southern Hemisphere warming was delayed by about 50 years, but this delay is not reflected in current climate simulations.

These findings point to the need to incorporate pre-twentieth century information in comprehensive assessments of human-induced climate change.