In this section we review the main causes and effects of climate change. Key solutions are reductions in greenhouse gas emissions from energy and agriculture, as well as capture or removal of carbon dioxide from industrial sources or the atmosphere, all of which may be spurred forward by carbon pricing. In this section we discuss adaptation and potential geoengineering solutions.
Industrial activity has emitted about 2200 billion tons (Gt) of carbon dioxide (CO2) into the atmosphere. Meeting internationally agreed goals of 2 °C or 1.5 °C of global warming over preindustrial levels would require that no more than 1500 or 580 Gt additional CO2 respectively, which would require a sharp reversal of the trend of growing emissions.
By sector, world greenhouse gas emissions are as follows as of 2016.
Following are overall greenhouse gas emissions from human activity.
Addressing the major sources of emissions requires replacing most of the world's current energy production with low carbon sources; reforming agriculture to reduce CO2 emissions from land use, methane from enteric fermentation, and nitrous oxide emissions; preventing the release of methane from decomposing organic matter in landfills, and phasing out the use of F-gases in refrigeration and air conditioning .
The International Panel on Climate Change has identified four future scenarios, based on how much global warming occurs. A brief summary of their impacts is as follows.
The World Health Organization estimates that climate change will be responsible for about 250,000 deaths per year in 2030 .
Climate change is also expected to cause deaths through coastal flooding and storms, but precise estimates of the number of deaths are lacking .
There are a number of potentially catastrophic outcomes from climate change whose likelihoods are unknown, including collapse of the West Antarctica Ice Sheet , release of large quantities of methane from the Arctic permafrost  or submerged methane hydrates , induced seismic events , and risks that are themselves unknown . The presence of unknown "fat tail" risks justifies a stronger response to climate change than consideration of only median or expected outcomes .
Over thousands of years, the oceans will absorb most excess carbon emissions. In the short term, carbon emissions travel as follows.
Update of carbon is the main cause of ocean acidification, or the long-term trend of falling ocean pH. Ocean acidification poses a threat to marine ecosystems.
Ocean acidification has negative impacts of fisheries, the ocean as a carbon sink, and the tourism, scentific, and cultural value of biodiversity . Losses has been estimated to reach $1 trillion per year by 2100 under projected CO2 emissions .
A main effect of soil uptake of CO2 is fertilization of plant growth. Based on satellite data, world vegetation growth may have increased 1.3-4.3% from 1982 to 2011, mostly as a result of CO2 fertilization. The following crop yield increases have been observed.
Climate adaptation refers to efforts intended to reduce harms from climate change, as opposed to mitigation, which is intended to reduce climate change itself. Worldwide, adaptation spending is a fraction of mitigation spending.
The Global Commission on Adaptation has found $1.8 trillion of adaptation projects expected to have net benefits .
Geoengineering can refer to any method of deliberately altering the Earth's climate or atmospheric composition. While direct air capture of CO2 from the atmosphere and bioenergy and carbon capture and sequestration (BECCS) are often considered froms of geoengineering, we consider them in our analysis of carbon capture and sequestration. Following is a summary of commonly considered geoengineering methods, their estimated costs, carbon removal or mitigation potential, limitations, and risks and drawbacks.
All proposed methods of geoengineering come with severe risk, limitation, and/or cost, though there is no reason to rule them out a priori.
There are several other methods sometimes discussed but not included above. The use of mirrors in orbit to reflect sunlight is at present impractical , as is irrigation of the Sahara Desert . Efforts to increase Earth's surface albedo, such as by painting roofs white, would make insignificant contribution . There are several methods under development that could directly remove greenhouse gases other than CO2, but data on their prospects is limited .
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