The impact of recent diurnally asymmetric warming on atmospheric dryness and terrestrial vegetation productivity

Our previous work reveals that daily maximum temperature (Tmax) over land has warmed at an accelerated rate in recent decades, contrasting with the faster warming of daily minimum temperature (Tmin) observed in earlier periods. This faster warming of Tmax relative to Tmin globally has led to a broadening of the diurnal temperature range (DTR), defined as the difference between Tmax and Tmin. However, the impacts of this accelerated daytime warming and increased DTR on the water and carbon cycles have remained largely unexplored.

Here, we show that the asymmetric warming rates between Tmax and Tmin have amplified the atmospheric vapor pressure deficit (VPD)—the difference between saturated vapor pressure (SVP) and actual vapor pressure (AVP). This amplification arises because a faster rise in Tmax compared to Tmin drives a larger SVP increase, due to the near-exponential relationship between temperature and SVP. Simultaneously, AVP is more strongly influenced by Tmin, as air is typically closer to saturation during the cooler nighttime hours. We quantified that the increase in DTR accounted for approximately 20% of the additional increase in global VPD over land.

We further investigated the response of terrestrial net primary production (NPP) to changes in DTR in the extratropical Northern Hemisphere over the past two decades. Our findings reveal divergent impacts of increased DTR on vegetation productivity in humid and arid zones, mirroring the contrasting effects of VPD on vegetation productivity in these regions. In humid zones, increases in DTR have promoted NPP, while in arid zones, the opposite effect is observed. This contrast is largely explained by the greater impact of accelerated daytime warming on increased VPD in arid zones, which inhibits NPP.

Additionally, we employed flux tower measurements to analyze the effects of DTR on net ecosystem carbon exchange (NEE, with negative values indicating net carbon uptake by the land) across various ecosystems. Our results demonstrate differential responses of ecosystems to changes in DTR. For example, in deciduous broadleaf forests, increases in DTR have had a dual negative impact on NEE, enhancing plant daytime photosynthesis driven by higher daytime temperatures while a more gradual rise in Tmin slows nighttime respiration increases. In evergreen needleleaf forests, the faster increase in Tmax relative to Tmin generally resulted in increased NEE, leading to a weak positive correlation between DTR and NEE.

Our findings provide compelling evidence that accelerated daytime warming over recent decades has significantly contributed to increased atmospheric dryness and has had divergent impacts on vegetation productivity in humid and arid zones. These results underscore the importance of understanding the responses of land surface hydrological processes, ecosystem productivity, and extreme events such as drought and wildfires to recent asymmetric warming dynamics.