aeroman

I would say we are largely past the second threshold too (that the warming is human caused). The last IPCC report had as the first statement in the summary for policymakers (from WG1 - the physical science group)

A.1 It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.

The previous report (from 2013) only said (and much further in)

Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.

The equivalent statement from AR4 (2007) was

The understanding of anthropogenic warming and cooling influences on climate has improved since the TAR, leading to very high confidence that the global average net effect of human activities since 1750 has been one of warming, ...

You could argue there is more of a question about what to do about it (e.g. try and mitigate climate change or just pay for the damanges). There is pretty good evidence at this point that mitigating the change through reducing CO2 emissions is a lot cheaper and comes with a host of other benefits (energy security, improved public health), but I can see wherer there might be arguments to have about this.

One thing I found really interesting abou the Graphcast paper (I appreciate this is not graphcast, but I think it is still relevant) is that it doesn't understand climate change. The model requires the training data to be recent to get the best quality projections.

While there are some factors that influence predictability in the weather forecast, as the fortran code is based on physics (at least in a broad sense), it doesn't suffer from those issues in the same way.

This doesn't mean that the ML forecasts are wrong (obviously), just different. Given the relative computational simplicity of running them, I wonder if the issue is not just expertise, but also understanding how they can best be used to generate reliable weather forecasts?

I think the one time bump could be a fair description in that is doesn't add to the long term warming rate in the same way GHGs do. The cumulative effect here is pretty much all realised within about 20 years (so not an instantaneous effect, but it could be considered a one time one?)

The wording of the article and the paper are a bit misleading here. It is definitely a one time even (although it plays out over 20 years)

The doubling of the rate of warming only applies for the first year or two. Based on this paper, over the period 2020-2030, the impact of ship fuel regulations is warming of about 0.12K. The long term temperature trend is around +0.19K over this 10 year period.

The mechanism is different from the impact of sulphur in the stratosphere (where there are no clouds), but it is the same as the mechanisms that have caused the majority of the aerosol cooling (a brightening of clouds).

The same ze of the cooling has long been uncertain though. It depends a lot on assumptions you make about the pre-industrial atmosphere,something that we don't have great observations of.

The numbers in this study need to be interpreted carefully, the way the authors presented them doesn't really help this.

The warming rate they quote (0.24K per decade) is the instantaneous warming, which decreases over time. The warming over 10 years is actually about half of this (0.12K). While still significant, it's not a doubling of the warming rate (which is around 0.19K per decade).

The forcing being equal to 80% of the heatuptake is also interesting, but we have not seen such a large step change in energy imbalance in the Earth system. This doesn't mean they are wrong about the forcing estimate, but it does not mean that 80% of the warming since 2020 has been due to ship fuel regulations either.

Water vapour is a very strong infrared absorber, so much so that it is more important for the greenhouse effect than CO2. However, it is usually very difficult to change the amount of water vapour in the atmosphere as a whole (it just rains out).

The stratosphere cycles through much slower than the troposphere (there are very few clouds, for example). That means that if you put water there, it can stay there for a lot longer, increasing the total amount of water in the atmosphere and warming the climate.

I'll also say that we expect water vapour to increase as temperature increases, which is a positive feedback in the climate system,increasing the warming beyond that of CO2.this is known as the water vapour feedback.

Maybe I should describe my job differently...

As a "cloud person", I just want to add a few things to the description of how clouds affect the climate (and why high clouds have a wamring effect).

All clouds are white, so they all reflect sunlight back into space (during the day), cooling the Earth.

All clouds are (almost) black in the infra-red, meaning the amount of energy they emit in the infra-red is determined by their temperature. Colder clouds emit less energy.

Almost all clouds are colder than the surface beneath them, which means they emit less infra-red energy to space than a clear day would. This reduces the amount of energy the Earth emits to space, so warming the climate.

High clouds are colder than low clouds, so have a stronger warming effect.

In summary:

Low clouds - Reflect sunlight (cooling), don't trap much infra-red (little warming)- Net: Cooling effect

High clouds - Reflect sunlight (cooling), trap lots of infra-red (stronger warming) - Net: Warming effect