Records that include coverage of the full Arctic, such as those from NASA, Berkeley Earth and Copernicus / ECMWF, showed 2017 as the second warmest after 2016, while records with sparser coverage in that region showed 2017 as the third warmest behind 2016 and 2015. (carbonbrief.org)
While it is possible to retrospectively adjust temperature readings taken with non-standard instrumentation, this task is much harder when the network has very sparse coverage and descriptions of recording practices are patchy. (bom.gov.au)
The disadvantages of archeomagnetic data for global field reconstruction are their scattered distribution in time and space and their extremely sparse coverage of the southern hemisphere (Fig. 1). (swsc-journal.org)