Given current trends it seems that the RCP6 scenario will be the most realistic outcome of current development. This is more conservative than the 2 °C scenario much talked about in the press and at the Paris climate conference, with less ambitious efforts needed to achieve it. It still implies quite a bit of emissions reduction, especially after a supposed 2050 peak. (Emissions have to halve from the peak by 2100) It means that the radiative forcing values in the year 2100 will be around 4.5 W/m2 if so. From here some likely projections can be made.
- Global sea levels will be, on average, 32 to 63 cm higher than in the year 2000. So Florida will still be around, mostly. Some very low Pacific islands may disappear if no sea walls are constructed. The Dutch will probably need to increase their walls a bit.
- Average temperatures above the jet stream in the northern hemisphere will increase from 3 °C to 5 °C above the 20th century average. Toronto for example will probably be 3 °C to 4 °C hotter by 2100 than it was in the 20th century. So approximately the current climate of Denver with severer winters. And Denver in 2100 will be like present day Austin, TX with colder winters. Austin will probably become too hot to live in without air conditioning in the summer.
- Coastal cities beyond the tropics will see around a 2 °C to 3 °C average increase.
- The good news is that coastal cities in the tropics will still be habitable, with only an average increase of 1 °C to 2 °C. Thought they will become more unpleasant outdoors, and air conditioning will become used in the summer even at night.
- Inland cities in the tropics however will likely need air conditioning 24/7 for at least the summer to be comfortable, due to an average increase of 3 °C to 5 °C. And walking outdoors during the day may become unbearable without special clothing and arrangements, like Bedouins in the present day sand deserts such as the Sahara. Most of inland India would fall into this category.
- Increasing desertification unless counteracted by ’green walls’, etc.
- Ice caps will melt by a significant amount, though the north pole will likely have a bit of ice cover remaining.
- Some of the higher altitude glaciers in the tropics, such as the Himalayas and Andes will still be around, though much reduced in size. Marginal glaciers will likely all disappear.
- Impacts on marine life, vegetation, and land animals are not yet understood well enough to say.
- Certain areas of the world will likely see significantly visual changes, though other places will see quite little change. Large chunks of Miami are likely to disappear without a seawall for example, though continental inland areas will just be warmer.
- Measurable negative effects of CO2 on human performance begins at 600 ppm. The RCP6 scenario means global average CO2 at 670 ppm, with cities more around 700 ppm, and indoors likely 800+