As the world adopts climate policies and the price of clean energy falls, we have and will continue to move away from some of the worst climate outcomes of 4C+ warming. But this should not distract us from our ultimate goal of getting emissions to net-zero…
A decade ago the world seemed on track for a very dark climate future. Global emissions were increasing at 3% per year, China was building a new coal plant every three days, and the idea that emissions could double or triple by 2100 did not seem that far-fetched.
Today things have changed markedly. Global coal use peaked back in 2013, and the International Energy Agency’s (IEA) most recent World Energy Outlook suggests that coal is now in “structural decline.” Global emissions are still increasing, but at a rate of only 1% per year.
Clean energy is the cheapest new form of generation in many countries, and the recent IPCC 6th Assessment Report acknowledges that “the likelihood of high emission scenarios such as RCP8.5 or SSP5-8.5 is considered low in light of recent developments in the energy sector.”
Thirty-two countries have absolutely decoupled their emissions from economic growth, and global emissions may begin to fall this decade as countries are increasingly committing to more ambitious near-term and long-term climate policies.…
Countries responsible for over two thirds of global emissions – including the US, China, EU, UK, Japan, South Korea, and others have made commitments to reach net-zero emissions by 2050 or 2060.
At the same time, the world is not on track today to reduce emissions rapidly enough to meet the Paris Agreement goal of limiting warming to well-below 2ºC. The world is currently on track for a central estimate of just under 3ºC warming by 2100 under policies in place today.
If we include near-term 2030s commitments made by countries under the Paris Agreement, this puts us on track for around 2.4ºC warming. Including mid-century net-zero commitments puts us on track for right around 2ºC.…
However, these long-commitments are in some cases stated goals rather than codified in legislation; the translation of long-term commitments into near-term goals will go a long way toward demonstrating how seriously we should take these targets.
These future warming estimates come with some large uncertainties; we do not fully understand how the climate will respond to our emissions, or precisely how the ability of the land and oceans to take up a portion of our emissions will change as the world warms.
These uncertainties are not our friend; while our central estimate of warming from current policies is 3ºC by 2100, we cannot fully rule out a chance of 4ºC if we get unlucky (or as little as 2ºC if we get lucky).
There has, thankfully, been some progress in narrowing this uncertainty in recent years. The recent IPCC 6th Assessment Report for the first time meaningfully narrowed the range of climate sensitivity.
It is useful to both celebrate progress while acknowledging how far we still have to go. We never needed a 5ºC world as a counterfactual to motivate limiting warming to well-below 2ºC.
As we move down the path toward mitigating climate change we will necessarily limit the range of possible future outcomes; the path we take need not distract us from our ultimate goal of getting emissions down to net-zero.
At the same time, we need to ensure that future emissions scenarios reflect the world that is, rather than what might have been if the progress of the last decade was erased. It is time to focus more on the negative impacts of a 3ºC world – not the RCP8.5 nightmares of a 5ºC one.

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More from @hausfath

2 Sep
For every 1C of warming the world experiences, saturated air contains 7 percent more water vapor on average.
Per the IPCC AR6: "The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area for which observational data are sufficient for trend analysis (high confidence), and human-induced climate change is likely the main driver."
"Event attribution studies and physical understanding indicate that human-induced climate change increases heavy precipitation associated with tropical cyclones (high confidence)."
Read 6 tweets
27 Aug
One under-appreciated finding in the IPCC AR6 is a lot more certainty around future warming.

Previously IPCC only gave "likely" warming ranges (e.g. a 2 in 3 chance of falling in the range). New report gives "very likely" 9 in 10 ranges. Here is a rough like-to-like comparison: Image
The IPCC AR5 future warming projections were nominally based on the 90th percentile of CMIP5 models, but the assessed range of climate sensitivity was much wider than the range in CMIP5 models, so these were treated "likely" (66th percentile) ranges.
The AR6, on the other hand, bases its warming projections on a combination of observationally-constrained CMIP6 models and a simple energy balance model using the new transient climate response (TCR) and equilibrium climate sensitivity (ECS) values in the report. Image
Read 9 tweets
19 Aug
Theres been a lot of debate about which simple metrics are best when comparing climate impacts of CH4 and CO2. The new IPCC AR6 report has a great figure (7.22) that compares different approaches. Ultimately, you want to come as close as possible to the actual temperature (GSAT):
Its pretty clear the optimal approach is GWP*. However, GWP* is a bit more complex and lacks constant equivalence (e.g. CH4 is always x times worse than CO2). Unfortunately given differing atmospheric lifetimes constant equivalence is not very accurate.…
If you are going to use GWP-20 or GWP-100, the IPCC AR6 figure suggests that GWP-20 will significantly overestimate near-term warming of continued emissions of CH4, while GWP-100 will modestly underestimate warming.
Read 5 tweets
11 Aug
The recent IPCC report had a big focus on methane (CH4) – and rightly so. We should work to cut methane emissions quickly, but not at the expense of cutting CO2.

Methane is temporary, while CO2 is forever.

A quick thread: 1/13
Methane is a strong greenhouse gas – over 100x more effective at trapping heat than CO2 while its in the atmosphere. Its responsible for around 28% of positive radiative forcing (and historical warming). 2/
However, methane has a short atmospheric lifetime. Most of the methane we emit this year will be gone from the atmosphere in around a decade. Methane interacts with hydroxyl radicals (OH) in the atmosphere, and ultimately breaks down into (mostly) CO2 and H2O. 3/
Read 14 tweets
11 Aug
There's grim news in the IPCC report, but also reasons for hope. We're flattening the curve of future emissions, and the darkest climate futures a decade ago are much less likely now. We can both celebrate progress and acknowledge how far we have to go:… 1/
A decade ago the world seemed on track for a particularly grim climate future. China was building a new coal plant every three days; global emissions were increasing at a rate of 3% per year and increased by 31% between 2001 and 2010. 2/
Scenarios where global carbon emissions tripled by the end of the 21st century with coal use increasing sixfold seemed plausible to many. Researchers argued that “business as usual” would likely lead to a world 4ºC or 5ºC above pre-industrial levels by 2100. 3/
Read 18 tweets
10 Aug
In my latest piece at @CarbonBrief, I take a deep dive into what the IPCC AR6 says about when the world will likely pass 1.5C and 2C, and how the new estimates of the remaining carbon budgets compare to those in the 2018 IPCC SR15 special report:… 1/14
When we talk about passing a particular warming level like 1.5C or 2C we are not referring to an individual year (or month). Any given year may be ~0.2C warmer or cooler than average due to natural variability from El Nino and La Nina events. 2/
The AR6 reports on the 20-year period in which temps exceed 1.5C (e.g. 2021-2040), and suggests using the midpoint of that range (e.g. 2030) as the year when long-term average temps passes that level. We used a similar approach in an earlier analysis:… 3/
Read 14 tweets

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