Ozone plays a central role in atmospheric chemistry: "Ozone affects climate and climate affects ozone". Ozone plays several different roles in the Earth’s atmosphere. In the high-altitude region of the stratosphere, ozone acts to shield the planet from harmful ultraviolet radiation. In the lower portion of the atmosphere (the troposphere), ozone can damage human health, crops and ecosystems. Ozone is also a greenhouse gas and contributes to global warming. In the Northern Hemisphere, ozone levels in the troposphere have increased by 35 per cent over the past century, with detrimental impacts on forest and agricultural productivity, even when productivity has been stimulated by increased carbon dioxide levels. In addition to reducing productivity, increased tropospheric ozone levels could alter terrestrial carbon cycling by lowering the quantity and quality of carbon inputs into plants and soils. In a global assessment of the impact of ozone on climate warming, scientists at the NASA Goddard Institute for Space Studies (GISS),estimated that tropospheric ozone was responsible for one-third to half of the observed warming trend in the Arctic during winter and spring seasons over the past 100 years. Furthermore, global warming was reported to stimulate the production of chemical precursors involved in ozone-generating photochemical reactions, thus contributing to the rise in its surface levels and indirectly stimulating climate warming.