Recently we've seen a vibrant debate on when the world will firmly pass 1.5C.

Over at @CarbonBrief I weigh in with a new analysis, finding that it will most likely occur in the late 2020s or early 2030s in a world where emissions do not rapidly decrease. carbonbrief.org/analysis-what-…
Global temperatures in any given year reflect short-term natural variability on top of longer-term human-driven warming. For example, a big El Niño or La Niña event can result in global temperatures up to 0.2C warmer or cooler, respectively, than they would otherwise be.

There is something of a genre of very online individuals™ discovering stratospheric aerosol injection and proclaiming it as a low-cost solution to climate change. Spoiler alert: its not.

In this case the thread uses a bunch of my figures so its worth responding.

https://twitter.com/cremieuxrecueil/status/1794231057815867807

Climate change is driven primarily by our emissions of carbon dioxide. We've emitted a lot of CO2: around 2.5 trillion tons since 1750, or the weight of the the biosphere and everything humans have ever built combined theclimatebrink.com/p/the-staggeri…

The first quarter of the year is off to an exceptionally warm start, as I discuss in a new Q1 State of the Climate Report over at @CarbonBrief:
⬆️ Warmest Jan, Feb, March, and April (to date) by ~0.1C
⬆️ 2024 on track to be warmest or second warmest year carbonbrief.org/state-of-the-c…
Based on first three months of the year and the current El Nino / La Nina forecast, we expect 2024 to be similar to or slightly warmer than 2023. With only 3 months in we can't know precisely where the year will end up, but its virtually certain to be at least the 2nd warmest:

February 2024 was the warmest February on record in the ERA5 dataset, at around 1.79C above preindustrial records.

It beat the prior record set during the 2016 super-El Nino by 0.12C: pulse.climate.copernicus.eu
It wasn't only the warmest February on record – this past month saw the largest anomaly (change from the 1850-1900 preindustrial baseline) at 1.79C of any month on record, beating out December (1.77C) and September (1.73C) 2023.

The past 12 months are 1.56C above preindustrial.

Despite today's grim milestone with the world passing 1.5C over the past 12 months, I see some reasons for cautious climate hope.

We stand both on the brink of severe climate impacts, but also on the brink of a rapid energy transition away from fossil fuels. A decade ago global emissions were skyrocketing, and many thought we were heading toward a particularly dark climate future where the 21st century would be dominated by coal and global emissions would double or triple by 2100.

I've been working over the past year on a promising new pathway to remove carbon from the atmosphere: Enhanced Rock Weathering.

With today's $57 million offtake agreement with @LithosCarbon the approach has gone mainstream:

A thread on the science:frontierclimate.com/writing/lithos Over geologic time the amount of carbon in the atmosphere is determined largely by the balance of volcanism and weathering of silicate rocks (which largely come from volcanoes!).

Today around a billion tons of CO2 is removed from the atmosphere annually by natural weathering.

Global temperatures in October smashed the prior monthly record by 0.4C, and were ~1.7C above preindustrial levels.

It wasn't quite as gobsmacking as September, but still comes in as the second most anomalous month in what has been an exceptionally hot year already. Here is a comparison of October 2023 compared to all prior Octobers in the two leading reanalysis products: ERA5 and JRA-55. Note that HadCRUT5 is used here to help estimate warming since preindustrial:

The claim by Hansen et al today that climate sensitivity is 4.8°C ± 1.2°C per doubling CO2 is just as plausible as the claim by Cropper et al four days ago that its 2.8°C ± 0.8°C.

Across hundreds of different studies, and our best estimate remains somewhere between 2C and 5C. Given all the conflicting estimates, I'd strongly advise folks against glomming onto any single new study (particularly if it informs ones priors that sensitivity is high or low). Instead, we should synthesize all the different lines of evidence: agupubs.onlinelibrary.wiley.com/doi/full/10.10…

Our 2023 Q3 State of the Climate is out over at @CarbonBrief:
⬆️ Record global temps since June
⬆️ Virtually certain to be hottest year on record
⬆️ Record-setting ocean heat content
⬆️ October on track for record warmth
⬇️ Record low Antarctic sea ice
carbonbrief.org/state-of-the-c…
The past few months were exceptionally warm for the Earth's surface, with September smashing prior records by around 0.5C (the largest margin a new monthly record has been set):

The first global temperature data is in for the full month of September. This month was, in my professional opinion as a climate scientist – absolutely gobsmackingly bananas. JRA-55 beat the prior monthly record by over 0.5C, and was around 1.8C warmer than preindutrial levels. Here are monthly absolute temperatures (compared to anomalies). This September would not have been out of place as a typical July this decade in terms of global temperatures.

This is mind boggling silly. We’ve put 2.5 trillion tons of CO2 in the atmosphere. The amount of CO2 in the atmosphere has increased by 1.1 trillion tons after being relatively stable for millennia before we started emitting. It’s hard to get more clear-cut causality.

https://twitter.com/curryja/status/1706821548588630282

If you believe temperature rather than emissions are driving atmospheric CO2 increase you quickly run into a mass balance problem. We know both the oceans and biosphere are net CO2 sinks today (we measure them!). We know atmospheric accumulation is ~half of our emissions.

Its hard to overstate just how exceptionally high global temperatures are at the moment. They have blown past anything we've previously experienced by a huge margin.

Over at The Climate Brink, we try and visualize this summer of extremes in seven charts. theclimatebrink.com/p/visualizing-…
Daily global surface temperatures over the past week have been a full 1C above the recent 1991-2020 baseline period use in the JRA-55 reanalysis dataset, or around 1.9C above preindustrial.

August 2023 shattered prior August records by ~0.3C, with temperatures at 1.68C above the 1850-1900 period.

In our August update we find that 2023 as a whole is virtually certain to be the warmest year on record, with a roughly 50% chance of reaching 1.5C berkeleyearth.org/august-2023-te…
The Earth's average temperature has been at record setting levels for June, July, and August (and roughly tied prior records in May). September seems to be keeping up the trend of exceptionally warm temperatures so far: climatereanalyzer.org/clim/t2_daily/…

One of the most important findings in the recent IPCC report is that we ultimately determine how much warming will occur.

There is likely no warming "in the pipeline" once emissions get to zero. Rather, CO2 concentrations fall and temperatures stabilize https://t.co/SQl5waLZtRcarbonbrief.org/explainer-will…
Why does this happen? As emissions fall, land and ocean carbon sinks start taking up more CO2 than we emit, as they work through a backlog of our past emissions. This causes atmospheric CO2 to fall and (all things being equal) would cause cooling. bg.copernicus.org/articles/17/29…

The internet has been afire with speculation about a "termination shock" from the recent regulations on sulphur in marine fuel accelerating warming.

In a new @CarbonBrief post @piersforster and turn to climate models to help assess the potential impacts. carbonbrief.org/analysis-how-l… As a bit of background, sulfur emissions (primarily SO2) have a strong cooling effect on the climate, both through directly scatting incoming sunlight (direct effects) and enhancing cloud formation (indirect effects). SO2 masks about half a degree of global warming today:

There is something weird happening at the UNFCCC. They argue removing 100 tons of CO2 for a single year and then re-releasing it can effectively offset a ton of fossil CO2 emissions.

But this is completely counter to what we said in the IPCC AR6 report! unfccc.int/sites/default/… The IPCC report was quite clear that the amount of warming is a largely time-independent function of cumulative emissions, and maintaining long-term net-zero emissions is required to stabilize temperatures.

The world is no longer following the worst case emissions scenario. This is good news, but we are still far from on track to meet our climate goals.

In a new post over at The Climate Brink we dig into the evidence and explore where we are headed now: theclimatebrink.substack.com/p/emissions-ar… Global emissions are now more or less tracking RCP4.5; the is particularly true if we look at fossil CO2, which is the most important factor in long-term growth (as its responsible for 90%+ of future emissions in high-end scenarios).

Fascinating and timely paper by @bach_lennart, @_david_ho_, and colleagues on how to deal with air sea gas exchange uncertainties when assessing the effectiveness of ocean-based CDR. Sounds pretty nerdy, but its actually really important!

https://twitter.com/bach_lennart/status/1656613198894878720

There are lots of proposed approaches to ocean CDR, including ocean alkalinity enhancement, direct ocean removal of CO2, and macro-algae (kelp) sinking. None of these directly remove CO2 from the atmosphere, however, and we need to figure out how long that would take!

I often criticize the use of biosphere carbon removal to make neutralization claims for fossil CO2 given the fundamental mismatch in atmospheric/biosphere residence times.

But if you are trying to neutralize methane emissions, there is a case to be made for temporary CO2 removal This is because the residence time of biosphere carbon is actually not that dissimilar from the residence time of atmospheric CH4 before it oxidizes, so their resulting climate effects should in turn be similar (provided you account for their different forcing magnitudes):

Without warming, we would expect fewer and fewer new record high temperatures over time. In a new post over at The Climate Brink, @AndrewDessler explores why we are actually seeing more and more new records set (hint: the world is in fact warming). theclimatebrink.substack.com/p/why-are-ther… In the absence of warming we'd expect the odds of setting a new record to decrease by 1/n over time, where n is the length of the record to date. We see precisely this behavior in unforced climate model control runs:

I'm excited to announce the launch of a new substack with @AndrewDessler! We are calling it The Climate Brink, as we are on the brink of both severe climate change and an energy transition.

My first post explores the odds if 2024 will be above 1.5C! theclimatebrink.substack.com/p/will-global-… With a growing El Nino event on top of rapid warming over the last few decades, 2024 is shaping up to be a scorcher. To examine where it might end up, we start by removing the effect of El Nino and La Nina (collectively called ENSO) from the temperature record: