# Shortened duration and reduced area of frozen soil in the Northern Hemisphere

## TL;DR

Frozen soil in the Northern Hemisphere is thawing earlier and freezing later, reducing its duration and area over 40 years due to rising temperatures and snowpack changes.

## Contribution

First comprehensive analysis of soil freeze-thaw cycle changes across the Northern Hemisphere using multisource data fusion.

## Key findings

- Frozen soil duration decreased by 0.13 ± 0.04 days/year from 1979–2017.
- Annual frozen soil area declined by 4.9 × 10⁴ km²/year, driven by delayed freeze onset and earlier thaw.
- Eurasia experienced more drastic freeze-thaw changes than North America, especially at mid-latitudes and in Arctic regions.

## Abstract

The changes in near-surface soil freeze-thaw cycles (FTCs) are crucial to understanding the related hydrological and biological processes in terrestrial ecosystems under a changing climate. However, long-term dynamics of soil FTCs at the hemisphere scale and the underlying mechanisms are not well understood. In this study, the spatiotemporal patterns and main driving factors of soil FTCs across the Northern Hemisphere (NH) during 1979–2017 were analyzed using multisource data fusion and attribution approaches. Our results showed that the duration and the annual mean area of frozen soil in the NH decreased significantly at rates of 0.13 ± 0.04 days/year and 4.9 × 104 km2/year, respectively, over the past 40 years. These were mainly because the date of frozen soil onset was significantly delayed by 0.1 ± 0.02 days/year, while the end of freezing and onset of thawing were substantially advanced by 0.21 ± 0.02 and 0.15 ± 0.03 days/year, respectively. Moreover, the interannual FTC changes were more drastic in Eurasia than in North America, especially at mid-latitudes (30°–45° N) and in Arctic regions (>75° N). More importantly, our results highlighted that near-surface air temperature (Ta) and snowpack are the main driving factors of the spatiotemporal variations in soil FTCs. Furthermore, our results suggested that the long-term dynamics of soil FTCs at the hemisphere scale should be considered in terrestrial biosphere models to reduce uncertainties in future simulations.

•Patterns and driving factors of soil FTCs across the NH during 1979–2017 were analyzed•The duration and annual mean area of frozen soil decreased significantly•Interannual FTC changes were more drastic in Eurasia than in North America•Near-surface air temperature and snow cover are the main factors in these variations

Patterns and driving factors of soil FTCs across the NH during 1979–2017 were analyzed

The duration and annual mean area of frozen soil decreased significantly

Interannual FTC changes were more drastic in Eurasia than in North America

Near-surface air temperature and snow cover are the main factors in these variations