Representatives from more than 190 countries will travel to Paris next week in emissions-belching vehicles to dine on five-course meals and talk about creating a process to reduce greenhouse gases over time. Then 170 of them will ignore it while the ones with few CO2 emissions will claim they are doing their part.
An economics paper in Science estimates that the Paris pledges can reduce the probability of the highest levels of warming, and increase the probability of limiting global warming to 2 degrees Celsius, if they are implemented and numerical models are accurate.
The models are based on Intended Nationally Determined Contributions, based on the years 2025 or 2030, and those are chosen by circumstances. China, for example, is allowed to emit unlimited emissions until 2030 after an agreement with US President Barack Obama, while the US agreed to reduce emissions in 2025 by 26-28 percent of 2005 levels.
If China gets what it wants under the umbrella of claiming to be a developing nation, and declaring they would ignore any other policy, and if everyone else cuts back, estimates of a range of increases in temperature show the cutbacks might work.
Climate scientists have focused on the importance of the 2 degree limit, but economist Allen Fawcett of the U.S. Environmental Protection Agency, the lead author of the paper, and colleagues assessed uncertainty in the climate change system from a risk management perspective. They analyzed the full range of temperatures the INDCs might attain, and determined the odds for achieving each of those temperatures. To determine odds, they modeled the future climate hundreds of times to find the range of temperatures these various conditions produce.
"It's not just about 2 degrees," said lead author Gokul Iyer of the Joint Global Change Research Institute. "It is also important to understand what the INDCs imply for the worst levels of climate change."
If countries do nothing to reduce emissions, they say the earth has almost no chance of staying under the 2 degree limit, and it is estimated that the temperature increase could even exceed 4 degrees. They found that INDCs and the future abatement enabled by Paris introduce a chance of meeting the 2 degree target, and greatly reduce the chance that warming exceeds 4 degrees. The extent to which the odds are improved depends on how much emissions limits are tightened in future pledges after 2030.
The model they used is the Global Change Assessment Model or GCAM that includes energy, economy, agriculture and other systems. The GCAM model produced numbers for global greenhouse gas emissions, which the team then fed into a climate model called Model for the Assessment of Greenhouse-gas Induced Climate Change or MAGICC. Running the simulations for each scenario 600 times resulted in a range of temperatures for the year 2100, which the team converted into probabilities.
