There was a nice article in New Scientist about the uncertainties of climate change here -
https://www.newscientist.com/article...e-really-know/
Unfortunately only available on subscription but here's the opening;
Quote:
TWELVE years to save the planet. Warming of 3°C, or perhaps 5°C if we don’t take drastic action now. Sea level rise of 0.3 metres by 2100 – or is it 3 metres?
Just about every article you’ll read about climate change is full of numbers, starting with 1.5°C, the number that we are told represents the maximum temperature rise we can allow and still avoid the worst effects of global warming.
Except it isn’t – and that is just the beginning of the confusion. No two numbers from climate change studies ever seem to agree. Even climate scientists are often baffled by the figures other researchers come up with.
Climate change deniers seize on the uncertainty as evidence that the underlying science is wrong. It’s not. It is just complex, as messy, real-world science is. The biggest uncertainty by far is us, namely what exactly we do over the next century. And the uncertainty cuts both ways: we could be underestimating how fast the world will warm and what the effects will be.
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I do subscribe so I can paraphrase the questions;
How much has the planet warmed already?
The Paris accord has set a limit of no more than a 1.5°C rise but how much have temperatures risen already? This should be easy but it isn't. Global records started about 1850 but there are some uncertainties. For example, land temperatures tend to be measured at 2m above ground while sea temperatures are measured at sea level. Also, we cannot measure the temperature of the arctic as the annual sea ice comes and goes.
The Met Office records and models leave the arctic out while NASAs estimate this. Also NASAs models estimate a pre-1850 rise of 0.2°C. Rolling this up, the Met Office says the planet has warmed by 0.9°C since 1850 while NASA says the planet has warmed by 1.2°C. It doesn't sound like much but is a big fraction when trying t limit warming by 1.5°C
What is a safe limit for warming?
No one really knows. There are suggested tipping points such as interruption of the north atlantic current but nobody knows for sure when anf if these events might happen.
We can also mitigate some of the effects of climate change by changing building codes and not building by the sea or in low lying areas.
When are we set to pass the 1.5°C limit?
Current trends suggest we will cross the line in the 2020s but we may cross the line and drop a little with an El Nino event. Long term constant above 1.5°C is estimated to be the 2040s.
There may be tipping point events that could rapidly change the rate of climate change such as the release of methane from permafrost melts or die back of the Amazon. Carbon capture technologies may reduce the ate of change.
How much warming does CO2 cause?
This is the toughest question when you try and model an entire planet. A small increase in temperature due to CO2 will increase water vapour in the atmosphere which is itself a greenhouse gas. If ice sheets melt, the reflective ice is replaced by dark ground which itself be warmed directly by the sun.
However, burning fossil fuels can release climate cooling compounds like sulphur dioxide. The oceans may absorb more CO2 than we thought too.
There are so many variables that the annual estimate of CO2 emissions limits needed to keep at or below the 1.5°C varies from 258 to 570 gigatonnes
How high will the seas rise?
During the last intergacial period where temperatures were on average 1°C higher than in 1850, the sea level rose 6-9 metres. However, there is a question on how long this will take if it happened again due to uncertainty in how long it would take for the ice in Greenland and the west Antarctic to melt. Estimates vary between 0.3 and 3 metres by 2100.
In summary - modelling the climate of an entire planet is hard!