Now researchers have found that the response of the drylands in south-western North America is complex. The grasslands in the east of the region have tended to become greener while vegetation greenness in the shrublands west of the Rocky Mountains has decreased. What's more, the scientists found that grassland vegetation greenness was strongly dependent on seasonal precipitation with a time lag of less than two weeks. Many of the shrublands, in contrast, responded to seasonal precipitation less strongly and one to two months later.

"Warming climate and prolonged dry conditions across south-western North America have caused significant environmental impacts during the last several decades, such as increases in fire hazards and tree mortality rates, and [it's likely] that frequent drought will continue in the 21st century," Xiaoyang Zhang of Earth Resources Technology Inc at NOAA/NESDIS/STAR told environmentalresearchweb. "We expect that these climate changes could significantly affect the vegetation growth in this region."

Zhang and colleagues from NOAA/NESDIS/STAR, Boston University and the US Geological Survey discovered that vegetation growth trends were cyclical; greenness tended to increase from 1982 to 1993 but decreased from 1993 to 2007.

Since 1993 vegetation greenness has dropped by 6% in shrublands and 13% in grasslands. The range of greenness variation between severe drought years and optimal years was 16% in shrublands and 21% in grasslands.

"The cycle and magnitude of vegetation variation is closely correlated to both annual precipitation and the length of the dry season," said Zhang. "This suggests that the magnitude of vegetation growth variation could be quantified using precipitation and drought duration in this region."

Drylands make up 41% of land worldwide; Zhang reckons that the impact of drought stress on dryland ecosystems ranks among the greatest environmental challenges today. "Reduction in vegetation greenness has led to the decrease of vegetation photosynthesis and carbon uptake across south-western Northern America since 1993, and some western regions since 1982," he said. "Our results imply that overall carbon dioxide uptake has decreased slightly in shrublands while it has increased in grasslands during the last 26 years. However, this region has transitioned from carbon dioxide sink to source because of the prolonged drought period during the last 14 years."

According to Zhang, if drought events continue as predicted by climate models, the aridification trend will enhance the reduction of vegetation greenness and carbon uptake, and could lead to changes in ecological communities, altering species composition and enabling the spread of alien species. "[This] has implications for sustainable land management, forestry and agriculture in this region," he said. "The local pattern (state level) in drought stress on vegetation growth is critical for mitigating and adapting to the threat of climate changes. Our research can help the public to be aware that climate change affects vegetation growth differently in arid and semi-arid regions, which will in turn have a significant impact on the economy in the US."

Zhang and colleagues used NOAA's satellite data from the Advanced Very High Resolution Radiometer to analyse vegetation growth for 1981–2007. They also employed NOAA North American Regional Reanalysis data on annual precipitation and dry season length.

"We identified explicitly the dates of onsets of vegetation green-up and senescence, and then calculated the net greenness in each individual growing season," explained Zhang. "In this way, the extracted-long-term vegetation greenness is reliable and spatiotemporally comparable, which is less affected by the satellite systematic biases caused by factors such as orbit drift and sensor degradation."

The AVHRR data have a resolution of about 4 km. NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) data, on the other hand, have a spatial resolution of around 250–500 m but are only available since the year 2000. "We will use MODIS data to investigate in detail the influences of drought stress on vegetation growth in local regions, to establish the relationship between seasonal vegetation greenness and seasonal climate parameters – rainfall and temperature," said Zhang. "We will also quantify the impacts of the prolonged drought on carbon dioxide uptake and species invasions." When NOAA's Geostationary Operational Environmental Satellite-R Series (GOES-R) is launched in 2015, the team will employ the observations of America that it will take every five minutes.

The researchers reported their work in Environmental Research Letters (ERL).