“It is difficult to get a man to understand something when his salary depends upon his not understanding it.” – Upton Sinclair
I’m not going to spend time on the physics or the evidence of climate change. To be clear, we’re talking about a) the measurement of the physical changes in the biosphere, cryosphere and the oceans over the past 150 years or so; and b) the thesis, overwhelmingly supported by national and international scientific bodies, that the majority of these changes are a result of human activity, primarily the increase in CO2 in the atmosphere (and the oceans) as a result burning fossil carbon in the form of oil coal and gas.
The global response to the threat of anthropogenic climate change, which began in 1988, has focused on defining tolerable limits to the changes which will take place, and changing our behavior to try and remain within these limits. Since 1992, with the adoption of the UN Framework Convention on Climate Change, this has been defined broadly by the Convention’s Article 2, i.e., to avoid ‘dangerous’ climate change. Up until recently this was defined as keeping global mean temperature rise below 2°C above the pre-industrial average; this was re-adopted in the recently concluded Paris Agreement, with an additional proviso that countries should be “pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change”.
 UNFCCC Article 2: “The ultimate objective of this Convention and any related legal instruments that the Conference of the Parties may adopt is to achieve, in accordance with the relevant provisions of the Convention, stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner.” http://unfccc.int/files/essential_background/background_publications_htmlpdf/application/pdf/conveng.pdf
 Paris Agreement, Article 2.1(a) https://unfccc.int/files/essential_background/convention/application/pdf/english_paris_agreement.pdf