We start at 2012 (the most recent year of emissions data in the TIAM-Grantham energy systems model used in this analysis) and assume that countries meet their 2020 Cancun target if a single figure was provided in their Cancun Pledge or, if a range was submitted, that they meet the weak (i.e. less ambitious) end of their range. Emissions by country are then assumed to rise linearly between 2020 and its 2030s INDC estimated emissions figure.
The “No mitigation” scenario is based on the “Representative Concentration Pathway (RCP) 8.5” scenario from the IPCC’s latest (fifth) Assessment Report, updated to include recent global emissions estimates. assumes emissions remain static at the 2030 INDC estimated level of emissions thereafter. This simplification is presented as a possible interpretation of the ‘no backtracking’ or similar text proposed for the Paris agreement and would be in spite of projected growth in global population and standards of living. The “Strong further action to meet a 2°C target” scenario starts from the 2030 INDC estimated GHG emissions level and follows a pathway through the 21st century with cumulative CO2 emissions constrained at a level estimated to be consistent with a 50% chance of keeping global average temperature increase below 2°C by 2100. This is modelled according to a lowest cost emissions reduction pathway from a combination of the TIAM-Grantham energy systems model (for CO2 ), the IIASA GAINS model (for other major greenhouse gases) and the RCPs (for minor GHG).
The GHG emissions pathways are converted to temperature change using a version of the simplified climate model (MAGICC) configured to sample uncertainty in the climate’s response to GHG emissions. For the “No mitigation” and “Emissions capped at INDC level” scenarios, GHG emissions are fed into MAGICC to determine the temperature change pathways to 2100. For the “Strong further action to meet a 2°C target” scenario, an iterative approach is required such that the combined GHG emissions from the different models and sources achieve the 2°C goal (when calculated using MAGICC) in a lowest cost manner.
Global-scale impacts under a specific climate pathway for a given year are estimated by combining the distribution of global mean surface temperature change produced by the simple climate model with a set of “damage functions” which define relationships between global mean surface temperature and impact. Different damage functions represent different regional changes in temperature and rainfall associated with a given change in global mean surface temperature. The estimated distribution of impacts in a given year under a specific pathway therefore incorporates uncertainty in (i) the change in temperature under that pathway and (ii) the regional distribution of changes in climate. The values in the infographic chart are the median-of the distribution of impacts in a given year. The impacts across all scenarios assume a ‘middle-of-the-road’ population growth scenario (scenario SSP2) throughout the 21st century, with approximately 9 billion people in 2100.
|IMPACT||No mitigation (REF 8.5)||Emissions capped at INDC level (54 GtCO2eq)||Strong further action to meet 2°C target||No climate change|
|Water Stress: millions of people exposed to increased water stress (m)||1921||1700 (12% less than REF8.5)||1425 (26% less than REF8.5)||0|
|Cropland Decline: cropland with decline in suitability (thousand km2)||7604||5704 (25% less than REF8.5)||4508 (41% less than REF8.5)||0|
|Flooding: people exposed to river flooding (m/year)||145||86 (51% less than REF8.5)||58 (76% less than REF8.5, once ‘no climate change’ value removed)||30|
|Heatwaves : people exposed to heatwaves (m/year)||12184||4506 (63% less than REF8.5)||1387 (88% less than REF8.5)||42|
The infographic figure shows the effect of climate change as the difference between impacts under a climate scenario and the impacts with no climate change (reference baseline climate).