The case for a dramatic increase in temperature and all the attendant effects relies on extrapolating temperature data recorded over a long time at different places around the globe, indirect evidence from ice cores, tree rings, etc., and climate models which include the impact of an increase in the amount of infrared absorbing gasses in the atmosphere.
Although the earth has undergone many temperature cycles, the contention is that the present changes are not part of the natural evolution of the earth and atmosphere, but rather due to the direct actions of humans, principally the use of hydrocarbon fuels and the attendant increase of carbon dioxide in the atmosphere. The data shows that for the places records are kept that over the last 100 years there has been about two degrees Fahrenheit in temperature increase. But it has not been steady. Until 1920 there was little increase and form 1942 until 1978 there was again almost no increase. However, since 1980 there has been a steady increase, even though there is a year-to-year variation of about plus or minus about half a degree F. Arguably there have been episodes of basically a decade of cooling surrounded by years of greater warming. The net effect over the last 200 years is net warming.
Presumably the root cause is anthropogenic and because of an ever-increasing demand for the energy content of fossil fuels. And although not exclusively, much of the increases is focused on the CO2 increase in the atmosphere from the burning of fossil fuels.
Many climate computer models capture the trend of increasing temperature, but not the year-to-year or decade-to-decade variations. Although the physics employed in the models are fairly well known, the details and the interactions are much less known. The future is, from the models, increasingly worse, and some models show it goes from just and increase in warming to a “catastrophic” warming – a nonlinear increase in temperature. Part of the variation in prediction is due to not knowing if the use of fossil fuels will decrease or increase. And part is just inadequate knowledge of the interactions of a problem that has many more variables than just Carbon dioxide. Some of unknown variables are social.
For example, the primary source of methane is feed lots and bovine flatulence. As 3rd world countries become more prosperous, their desire for meat increases. Alas. But also there may be a reduction in meat as it become increasingly expensive. And a very important greenhouse gas is water vapor. As the atmosphere and oceans warms the amount of water vapor in the atmosphere increases as the saturation vapor pressure increases. The melting ice will locally change the salinity of the oceans and ocean currents may be altered. There is likely to be a cascading of effects both big and small in many details. Almost everything will feel some effect, from agriculture, aquaculture, habitat, etc. Most importantly the weather patterns will be certainly be affected, but in ways that are hard to predict – More/less storms, more/less rainfall, different storm paths, sea level rise and relocation of beaches inland from their present location, etc. Weather patterns will be altered and some areas will experience warmer temperatures and more or less rain while others cooler and/or less rain.
As the polar ice melts, the sea level will rise and habitat for animals from polar bears to seals will change. As the oceans acidify, the habitat in the oceans will change and coral will struggle to survive. Some low-lying islands will get smaller and perhaps go underwater. The forecast effects will certainly make a different would. But it will NOT turn into “Water World”.