A new study published in the scientific peer-reviewed journal, Climate, by 37 researchers from 18 countries suggests that current estimates of global warming are contaminated by urban warming biases.

The study also suggests that the solar activity estimates considered in the most recent reports by the UN’s Intergovernmental Panel on Climate Change (IPCC) likely underestimated the role of the Sun in global warming since the 19th century.

It is well-known that cities are warmer than the surrounding countryside. While urban areas only account for less than 4% of the global land surface, many of the weather stations used for calculating global temperatures are located in urban areas. For this reason, some scientists have been concerned that the current global warming estimates may have been contaminated by urban heat island effects. In their latest report, the IPCC estimated that urban warming accounted for less than 10% of global warming. However, this new study suggests that urban warming might account for up to 40% of the warming since 1850.