Phenology's Net Cooling Effect as Feedback to Global Warming

Recent decades have seen significant changes in land surface phenology, with earlier leaf development in northern ecosystems and diverse changes in autumn senescence, primarily attributed to climate change. These phenological changes feed back to Earth’s climate system by altering biogeochemical and biogeophysical processes at the land surface. However, little is known about the strength of these diverse effects on the Earth's energy balance, and whether their combination results in a net positive (warming) or negative (cooling) feedback to global warming.

Using a fully-coupled Earth system model (ESM) with an interactive global carbon cycle, we investigate the effects of land phenological changes on the Earth's energy balance and the subsequent biogeophysical and biogeochemical feedbacks. We prescribe transient shifts in leaf area index (LAI) in the ESM based on remote sensing estimates of phenological spring advancement (2.1 days per decade) and autumn delay (1.8 days per decade). Note these shifts are only prescribed for extratropical northern ecosystems, where robust phenological changes have been observed, however, the effect in the ESM is global including local and non-local effects. Our results provide a first quantification of the impact of these phenological changes on the processes affecting the Earth’s energy balance, namely, shortwave radiation through changes in surface albedo, surface sensible and latent heat fluxes, longwave surface emissions, ground heat flux, longwave radiation balance through greenhouse gases, and the overall radiative fluxes through cloud properties and planetary albedo.

We find that autumn LAI shifts have a stronger net effect than spring LAI shifts on the Earth's energy balance, and that these effects can compensate each other when they co-occur in the same year. Our simulations also reveal compensating effects between outgoing longwave and outgoing shortwave radiation at the top of the atmosphere, where the former points to a positive and the latter to a negative radiative forcing. Altogether, we report an average negative radiative forcing of 0.17 ± 0.1 W m^-2 for a 10-day lengthening of the growing season, resulting in a global mean surface temperature cooling of 0.1 ± 0.03 °C. The effect is more pronounced in simulations when spring advancement and delay in senescence are prescribed separately in the ESM, amounting to a negative radiative forcing of 0.24 ± 0.21 W m^-2 and 0.32 ± 0.25 W m^-2 for a 10-day lengthening of the growing season, respectively. These simulations suggest that phenological changes triggered by global warming result in a net negative feedback to global warming. Future research is needed to confirm this first quantification and to investigate the saturation of phenological responses to global warming, which could weaken this cooling feedback effect in the future.