The effects of global warming are the environmental and social changes caused (directly or indirectly) by human emissions of greenhouse gases. There is a scientific consensus that climate change is occurring, and that human activities are the primary driver.[1] Many impacts of climate change have already been observed, including glacier retreat,[2] changes in the timing of seasonal events[2] (e.g., earlier flowering of plants),[3] and changes in agricultural productivity.[2]
Future effects of climate change will vary depending on climate change policies[4] and social development.[5] The two main policies to address climate change are reducing human greenhouse gas emissions (climate change mitigation) and adapting to the impacts of climate change.[6] Geoengineering is another policy option.[6]
Near-term climate change policies could significantly affect long-term climate change impacts.[7][4] Stringent mitigation policies might be able to limit global warming (in 2100) to around 2 °C or below, relative to pre-industrial levels.[8] Without mitigation, increased energy demand and extensive use of fossil fuels[9] might lead to global warming of around 4 °C.[10][11] Higher magnitudes of global warming would be more difficult to adapt to,[12] and would increase the risk of negative impacts