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The amount of carbon stored in deadwood is equivalent to about 8% of global forest carbon stocks 1 . Deadwood decomposition is largely governed by climate [2][3][4][5] with decomposer groups, such as microbes and insects, contributing to variations in decomposition rates 2,6,7 . At the global scale, the contribution of insects to deadwood decomposition and carbon release remains poorly understood 7 . Here we present a field experiment of wood decomposition across 55 forest sites on six continents. We find that deadwood decomposition rates increase with temperature, with the strongest temperature effect at high precipitation levels. Precipitation affects decomposition rates negatively at low temperature and positively at high temperatures. As net effect, including direct consumption and indirect effects via interactions with microbes, insects accelerate decomposition in tropical forests (3.9% median mass loss per year).In temperate and boreal forests we find weak positive and negative effects with a median mass loss of 0.9% and -0.1% per year, respectively. Furthermore, we apply the experimentally derived decomposition function to a global map of deadwood carbon synthesised from empirical and remote sensing data. This allows for a first estimate of 10.9 ± 3.2 Pg yr -1 of carbon released from deadwood globally, with 93% originating from tropical forests. Globally, the net effect of insects accounts for a carbon flux of 3.2 ± 0.9 Pg yr -1 or 29% of the total carbon released from deadwood, which highlights the functional importance of insects for deadwood decomposition and the global carbon cycle.