Understanding past and future patterns of elevation-dependent climate change in mountains: a meta-analysis of global temperature and precipitation changes.

It is widely accepted that mountain and high elevation regions often show distinct trends in temperature and precipitation in a warmer world, in contrast to lowland regions. In the context of temperature, this phenomenon has been identified as elevation-dependent warming (EDW). Temperature trends can sometimes increase with elevation but this is not always so, and they may peak in a critical elevation band, or show more complex elevation profiles. This is due to a variety of mechanisms which may be responsible for the observed patterns, including snow albedo feedback, vegetation change, cloud and moisture patterns, aerosol forcing and to their interactions. In this study we examine the literature for studies which have assessed elevation profiles in recent warming rates, or have quantified temperature change in mountain regions in general. We find for the recent historical period (~1960-2010) that when comparing like with like (i.e. high elevation regions with adjacent low elevation regions) warming rates are nearly always more rapid on average at higher elevations. Warming rates have also increased over time, with more recent decades showing stronger warming. On a global scale there is no significant difference between mean warming rates in mountains and in all other areas. Thus, elevation-dependency within regions can be masked by differences in geographical location in global meta-analyses. Although there have been far fewer studies on changing precipitation gradients, we extend our meta-analysis to examine elevation profiles in precipitation change and in the solid/liquid precipitation ratio, where information is available. We examine the concept of the "adjacent lowland region" by comparing mountain and lowland change using temperatures and precipitation from global gridded observation-based and reanalysis datasets (e.g. CRU, ERA5, NCEP2), and defining mountains and adjacent lowlands in different ways. The choice of definition has a strong influence on the elevation-dependent relationships uncovered. Applying a similar method to consider future modelled change from GCMs shows that unlike past observed change, most model studies confirm enhanced warming at higher elevations and a decreased solid/liquid precipitation ratio in nearly all mountain ranges by the end of the 21^st century.