How are snowmelt rates actually changing across the Northern Hemisphere?

There is strong evidence that rising temperatures lead to less snow accumulation and to an earlier start of the melt season. This has an effect on the magnitude and timing of streamflow generated by snowmelt in spring. Higher temperatures should intuitively lead to faster snowmelt, but some studies suggest that melt rates might be slower in a warming world because the melt onset occurs earlier in the year when less energy is available for melt. Understanding of these changing snow dynamics is challenged by a lack of observations on water content of the snowpack, the Snow Water Equivalent (SWE). However, high quality observations of snow depth are generally more available in both space and time, even at higher elevations. Here we gather several datasets of long-term observed snow depth time series over the Northern Hemisphere, and convert them to SWE. We then investigate changes in total snowmelt, timing of snowmelt and melt rates for the period 1980-2020 over a ranger of climates and regions. Large decreases in total melt and earlier melt timing are widely observed. However, trends in snowmelt rates are generally weak and spatially inhomogeneous. Slower snowmelt in a warmer world occurs mostly on deep snowpacks that have been heavily depleted, but faster melt or no significant change in melt rate are observed too. We provide an analysis of the causes for the spatial and temporal variability in trends. We find that changes and trends can differ depending on the definition of melt rate and peak SWE. Strong warming generates large melt events during the late accumulation season, challenging the commonly used definition of peak SWE and making it harder to compare the snowmelt dynamics of the past and the current climate. We highlight that focusing only on melt rate change might mask important effects on melt timing and magnitude, because a proportional reduction in total melt and number of melt days can lead to no change in melt rate.