Considerable expansion of shrubs across the Arctic tundra has been observed in recent decades. These shrubs are thought to have a warming effect on permafrost by increasing snowpack thermal insulation, thereby limiting winter cooling and accelerating thaw. Here, we use ground temperature observations and heat transfer simulations to show that low shrubs can actually cool the ground in winter by providing a thermal bridge through the snowpack. Observations from unmanipulated herb tundra and shrub tundra sites on Bylot Island in the Canadian high Arctic reveal a 1.21 °C cooling effect between November and February. This is despite a snowpack that is twice as insulating in shrubs. The thermal bridging effect is reversed in spring when shrub branches absorb solar radiation and transfer heat to the ground. The overall thermal effect is likely to depend on snow and shrub characteristics and terrain aspect. The inclusion of these thermal bridging processes into climate models may have an important impact on projected greenhouse gas emissions by permafrost.

近几十年来，人们观察到北极苔原上的灌木大量扩张。这些灌木被认为通过增加积雪的隔热性对永久冻土产生变暖效应，从而限制冬季降温和加速解冻。在这里，我们使用地面温度观测和热传递模拟来表明，低灌木实际上可以通过在积雪中提供热桥来冷却地面。对加拿大高北极地区拜洛岛上未经处理的草本苔原和灌木苔原遗址的观察表明，11 月至 2 月之间有 1.21°C 的降温效应。尽管雪堆在灌木中的绝缘性是两倍。春季，当灌木枝条吸收太阳辐射并将热量传递到地面时，热桥效应会发生逆转。整体热效应可能取决于雪和灌木特征以及地形方面。将这些热桥过程纳入气候模型可能会对永久冻土的预计温室气体排放产生重要影响。