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I'm increasingly seeing people justify climate policy choices by saying "science says". This is, more often than not, a misuse or misunderstanding of what scientists are actually saying. For example, the @ExtinctionR movement claiming science says we need zero emissions by 2025.
Scientists can examine a range of potential future emission scenarios using climate models. We can assess some future damages associated with these scenarios, though its generally economists and energy system modelers who focus on the mitigation cost and pathways.
So what does science actually tell us? It tells us that the more warming we have, the larger the impacts on society will be. Based on this political leaders have agreed to a target to limit warming to "well below" 2C, with an aspirational goal of holding it to 1.5C.
Science tells us that to limit warming well below 2C (e.g. with a 66% chance of avoiding 2C) we can emit no more than 1170 gigatons of CO2, or roughly 28 years of current emissions. A 50% chance of 1.5C has a much smaller budget of 580 gigatons, or 14 years of current emissions.
How this budget is divvied up - and how much it is potentially expanded by negative emissions - is very much an open question. Energy system models show global emissions reaching zero between 2040 and 2050 in 1.5C scenarios, and around 2070 in 2C scenarios.
What the science does tell us is that if we want to limit warming to either 1.5C or 2C we need to start reducing emissions now, and that delays will make the challenge significantly harder (and quickly make the 1.5C target completely unachievable).
However, it also tells us that there are scenarios where emissions are reduced over multiple decades that are still consistent with stringent mitigation goals, at least if later century negative emissions are included.
Science also can't necessarily tell us if 1.5C or 2C is the optimal target from the standpoint of human welfare. Both the damages and the mitigation costs are highly uncertain.
We can clearly see severe impacts under high emission scenarios were the world warms 3C or 4C, but the marginal difference between both mitigation costs and impacts in 1.5C and 2C is tough to suss out.
Decarbonizing the whole world by 2040 would impose large costs, if its even possible. The choice of 1.5C or 2C as the target is one for policymakers based on the balance of impacts and costs (and equity), and there is not necessarily a clear answer that "science" tells us.
One might ask "why does it matter what goal we choose, if everyone agrees we need to take action today?". To an extent, it doesn't matter that much whether we aim for 2C or 1.5C as long as we are still increasing emissions.
But at the same time there is a world of difference between a 30-year decarbonization pathway where assets are retired at end of life, and a 10-year crash course where you'd have to prematurely retire a ton of capital, buy up existing cars, etc.:
To be clear: current mitigation and current commitments by countries are vastly inadequate to meet any of our climate targets. carbonbrief.org/unep-limiting-… 
