Two paths converge at the giant Paris climate conference: the political path and the scientific path. Based on the following discussion you can decide what you think will happen in Paris.

The political path had its origins in 1992 at the Rio Conference where the UN delegates decided that greenhouse (fossil fuel) emissions must be sharply reduced so that the average earth temperature does not rise more than 2 degrees C (3.5 F) above the 1750 level. Even given warnings then that a 2-degree temperature rise was too much for our earth to handle, the delegates fixed on the 2-degree limit, fearing their countries would never accept a lower target since this would require them to make greater sacrifices.

Now the 2-degree target is chiseled in stone. Thus the objective of the Paris Conference is to gain pledges from the 196 participating countries to reduce their emissions to keep the temperature from rising above the 2-degree target.

Note, however, that the pledges to be made in Paris are voluntary. Though scientists say that in the future only 500 billion metric tons of additional CO2 can safely be placed in our atmosphere, there is no chance that quotas adding up to the 500-ton limit will be assigned to the attending countries, or even that there will be any enforcement of pledges.

Now for the scientific path, starting with two fundamental scientific points. First, once CO2 is placed in the air it stays there in decreasing amounts for hundreds, even thousands, of years. So at any point of time, like now, there is a certain amount of warming that has been caused by the prior emissions which now total around 500Bt. These emissions have produced warming so far of about 1 degree C. Then we must add the continued warming in the future that the CO2 placed previously in the sky, and still remaining in the sky today, will produce over future years. This is estimated at 2.7degrees. Thus if emissions were magically terminated today, the world temperature would still rise by 2.7 degrees. Thus adding the 1 degree “so far” plus the “to come” future warming of 2.7 degrees, we get an expected total rise of 3.7 degrees because of the 500 tons of CO2 already placed in the air. This is almost double the UN target of 2 degrees.

Second, science can determine fairly well “what” will happen, but not exactly “when.” Precise evidence from the melting phase of the last ice age over thousands of years reveals 41 times when the earth’s temperature and ocean level rose abruptly in a matter of decades, and by many degrees and by many feet. Earlier views that the climate always changes gradually and gives humans advance warming have been disproved. Nature works in lurches. Melting permafrost released billions of tons of CO2 into the atmosphere. This caused runaway warming which in turn caused the ice caps to melt and decay causing oceans to rise precipitously. Once such a runaway cycle starts, it accelerates relentlessly until the icepacks and the permafrost are all melted. Thus scientists conclude that it is foolhardy for mankind to rapidly force climate change. There is no telling when a dangerous lurch might be launched.

So the scientific path leading to Paris says:

The safe target to prevent runaway climate change is maybe 1 degree or perhaps 1.5 degrees, but not 2. The Arctic has already warmed some 4 degrees. The North and South Ice Caps are flowing larger and larger rivers of melt water into the oceans. Thus one can say that the world is already in jeopardy, even before the start of the Paris Conference. There are forecasts that if warming continues at the present rate there may be as many as 50 million migrants fleeing heat, newly made deserts, water shortages and severe crop failures during the coming decades.

What to do? The Paris objective must be to eliminate fossil fuels as quickly as possible, replacing them with wind, solar and nuclear energy. James Hansen, a leading world climate scientist, argues persuasively that quickly the price of the renewable, non-fossil fuels must be made cheaper than the fossil fuels of coal and petroleum. This, he argues, can be accomplished by creating a worldwide tax on fossil fuels which is increased each year, making producing renewable fuels profitable, while creating disincentives for carbon-based fuels. In short, make green fuels cheaper than carbon fuels and the market will do the job for us.

Roy Wehrle of Springfield is professor emeritus in economics and climate science at University of Illinois Springfield.