Global terrestrial stilling: does Earth’s greening play a role?

Abstract

Previous studies have documented that surface wind speed (u) has been increasing over the ocean but decreasing over land for the past several decades. The decreasing u at the surface over land has been referred to as terrestrial stilling. A plausible hypothesis for terrestrial stilling is an increase in surface roughness associated with changes in land surface (e.g. enhanced vegetation growth, landscape fragmentation or urbanization). One of the most widespread land surface changes is enhanced vegetation leaf area index (LAI) known as greening, particularly over the middle to high latitudes of the Northern Hemisphere where strong stilling is observed from weather station data. In this study, we examine the hypothesis that enhanced vegetation LAI is a key driver of global terrestrial stilling. We first characterized the trend in u over the ocean using long-term satellite altimeter measurements, and the trend in u over land using continuous wind records from 4305 in situ meteorological stations. We then performed initial condition ensemble Atmospheric Model Intercomparison Project-type simulations using two state-of-the-art Earth system models (IPSL-CM and CESM) to isolate the response of u to the historical increase in LAI (representing the greening) for the period 1982-2011. Both models, forced with observed sea surface temperature and sea ice and with LAI from satellite observation, captured the observed strengthening of Pacific trade winds and Southern Ocean westerly winds. However, these simulations did not reproduce the weakening of surface winds over land as significantly as it appears in the observations (−0.006 m s −1 versus −0.198 m s −1 during 1982-2011), indicating that enhanced LAI (greening) is not a dominant driver for terrestrial stilling.

An analysis of the urbanization contribution to observed terrestrial stilling in the Beijing–Tianjin–Hebei region of China

“…Despite these limitations, this study rejected the hypothesis that urbanization was a dominant driver of regional terrestrial stilling in the urban areas of the Beijing-Tianjin-Hebei region. Given the minor role of vegetation growth in NSWS change (Zeng et al 2018), it appears that the observed decrease in NSWS over this region was probably driven by the natural internal variability of the climate system and/or large-scale anthropogenic climate changes. In the case of JRA-55 reanalysis, we found that a clear understanding of the interactions and couplings among the surface layer, planetary boundary layer and free atmosphere was a prerequisite to accurately reproduce historical and future NSWS changes.…”

“…Beijing-Tianjin-Hebei, Yangtze River Delta, Pearl River Delta, and North China Plain) were relatively small. At the global scale, besides northern China, rapid declines in NSWS were also observed in eastern North America, northeastern Europe, Siberia and even over the Tibetan Plateau (You et al 2014, Zeng et al 2018, Zhang et al 2019b, where urbanization is very slow or negligible. A recent study indicated that decadal-scale variations of NSWS was dictated by internal decadal climate variabilities, rather than by the local impact of urbanization (Zeng et al 2019).…”

“…However, it should be noted that an increase in surface roughness may not be primarily or solely caused by vegetation. Using three atmospheric global climate models forced by historical sea surface temperature and sea ice records, Zeng et al (2018) found that the simulations with realistic vegetation changes could not reproduce the observed weakening of NSWS over land, including over China, implying that enhanced vegetation growth was not a primary driver for NSWS changes there. Recognizing the minor role of vegetation growth in the observed declines in NSWS in China, coupled with the knowledge that urbanization can also increase surface roughness, led us to assess the impact of urbanization on terrestrial stilling in rapidly urbanizing regions.…”

Urbanization has stronger impacts than regional climate change on wind stilling: a lesson from South Korea

“…Wind stilling has been reported as an important phenomenon in recent decades (Roderick et al 2007, Vautard et al 2010 as the observed land-surface wind speed has decreased in a number of regions (Bichet et al 2012, Wu et al 2018, Zeng et al 2018. By analyzing the observations at 822 surface weather stations, Vautard et al (2010) showed that the surface wind speed has been reduced by 5%-15% over almost all continents in the Northern Hemisphere (NH) midlatitudes over .…”

“…A recent study also showed a decrease in surface wind speeds and wind power potential in most of NH over the past four decades (Tian et al 2019) exerting adverse impacts on windgenerated electricity (Sherman et al 2017) and the dispersion of air pollutants (Li et al 2019b), reducing evaporation (McVicar et al 2012), and affecting soil erosion (Zhang et al 2019). However, the mechanisms related to the observed wind stilling are not well understood (Wu et al 2018, Zeng et al 2018. Moreover, climate models have limited ability for simulating the long-term trend in surface wind speeds (Jiang et al 2017, Tian et al 2019, indicating the need for a clear understanding of the mechanisms of wind stilling.…”