A new analysis by Camilo Mora, et al from University of Hawaii, projects the dates of climate departure,
when the projected mean climate of a given location moves to a state continuously outside the bounds of historical variability
compared to 1860 to 2005. This is the date when the coldest year is warmer than the warmest year in our past.
Year of climate departure on our current emissions path
Worldwide the average year for climate departure is 2047. The effects on the tropics are more serious, not because temperature increases will be larger, but because normal variability is small. The mean year for the tropics under this scenario is 2038, later (2053) for cities outside the tropics.
One consequence is that poor areas appear to be suffering first, with one city in Indonesia likely to see climate departure within the decade (2020). Lagos (2029) and Mexico City (2031) are projected to reach this point within 2 decades; both have populations over 20 million. A number of large cities in the United States are expected to see temperatures rise to historically unknown levels by the late 40s and 50s. This is for the high greenhouse gas emissions scenario RCP8.5* (not the highest).
The Mora Lab site has projected dates for many more cities; check them out if your city isn’t listed.
The low greenhouse gas emissions scenario (RCP4.5*) delays average climate departure to 2069 but does not prevent it. The first affected area of Indonesia will see the date move up to 2025. Lagos won’t see climate departure until 2043, and Mexico City until 2050. A number of large cities in the US see their own dates delayed until the 2070s or even later.
The Mora paper discusses the effects on the ocean, which will see climate departure in the next decade or so. When considering temperature and acidity together, the ocean moved outside its normal variability in 2008.
Table Species group year of climate departure
* Some more information on RCP4.5 and RCP8.5
Intergovernmental Panel on Climate Change has new scenarios, called Representative Concentration Pathways. The number, 4.5 or 8.5, represents the warming in 2100 in watts/sq meter, or W/m2. If the number is positive, that means there is still a net warming of Earth. The most optimistic scenario provided by IPCC in the latest report is RCP2.5, where the net flow of energy in is at the rate of 2.5 watts/sq meter, down from a peak a few decades from now at 3 W/m2. This scenario, considered perhaps too optimistic, is likely to keep temperature increase below 2°C. A more reasonable low estimate is RCP4.5, which will produce a temperature increase by the last two decade of the 21st century of close to 3°C over preindustrial times, and RCP8.5, our current trajectory, which could produce a temperature increase closer to 5°C.
RCP4.5 allows us to emit about 780 billion tons of carbon (carbon dioxide) between 2012 and 2100. RCP allows 1685 billion tons of carbon in the same period. (Multiply quantity of carbon by 44/12 to get quantity of carbon dioxide.) Carbon emissions in 2012 were 9.7 billion tons. Counting land use change, the number is even higher. The average rate of increase was 3.2%/year from 2000 to 2009 (doubling time = 22 years).