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But the models disagree on a lot else. Lots of further exploration of these responses, including a bunch of maps of temperature and precipitation changes, in the manuscript!

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Charlie Koven @ckoven.bsky.social · 16h

For example, the ESMs all agree that, at a given global temperature, global precipitation will be higher in both cases than at the point where the experiments diverge, and they mostly agree that the exceedance-and-return responses will be amplified relative to the zero-emissions responses.

This figure shows multiple panels of the same type of plot, with change in temperature on the x axis and change in precipitation on the y axis. Each panel shows the responses for a different Earth system model (ESM). In each panel there are three curves: (1) a black curve, for the experiment with continued positive CO2 emissions, in which temperature and precipitation both increase and follow a relatively straight line, (2) a blue curve, for the experiment with zero emissions, in which temperature is roughly constant, though increases or decreases slightly in different models, but in all cases the precipitation increases at a given temperature so that the blue curves extend above the black curves, and (3) a green curve, representing the experiment that reverses fgrom positive to negative emissions, which lies roughly parallel to but above the black curve. The slopes and offsets between the curves differ between each ESM, but the overall pattern is the same for all ESMS.

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Charlie Koven @ckoven.bsky.social · 16h

The pathways therefore arrive at different points along the tradeoff between declining atmospheric CO2 and declining ocean heat uptake that gives rise to a near-zero Zero Emission Commitment, which has implications for other aspects of climate.

This is a diagram showing different energy fluxes for the three experiments. There are three columns, and in each column there are two boxes, labelled "atmosphere" and "ocean", and two arrows, labeled "Relative ocean heat flux", from the atmosphere to the ocean, and "Relative longwave energy to space", going up from the atmosphere. In the left column, labelled "Before Zero Emissions" the atmosphere box is labeled "High CO2" and is dark grey, indicating it is relatively more opaque because of the high CO2, with a relatively small arrow going to space and a larger arrow of heat going into the ocean. In the middle column, labelled "After zero emissions", the atmosphere box is labelled "Lower CO2", the box is a lighter grey, the arrow of energy to space is larger and the arrow of heat into the ocean is smaller. On the right, labelled "After exceedance and return", the atmosphere box is labelled "Lowest CO2", the arrow of energy going to space is largest, and the arrow of heat going into the ocean is smallest, and with unclear directionality.

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Charlie Koven @ckoven.bsky.social · 16h

What's interesting is that, relative to the point where the experiments diverge, global mean temperatures are roughly the same, but CO2 concentrations are notably lower under first positive and then negative emissions than under zero emissions, for the same level of total cumulative CO2 emissions.

this plot shows temperature trajectories along the three experiments, relative to the values at the year-100 timepoint where they diverge. there is also a box plot showing the value at the year-200 timepoint where the experiments reconverge. The mean values are essentially the same (a roughly zero degree temperature change), though the width of the boxes is larger for the experiment that exceeds and returns than for the zero-emissions experiment.

This plot shows the CO2 concentrations that correspond to the temperature trajectories in the prior plot. A bunch of blue curves show gradual CO2 declines under zero emissions, relative to the point where the experiments diverge. A bunch of green lines first increase, then peak and decline from the point of divergence. At year 200, the green lines are below the blue lines, which is also shown in the box plot to the right that shows that CO2 concentrations are lower in year 200 for the experiment that exceeds and then returns to a given cumulative CO2 level than for the experiment that reaches zero emissions at that same level and over the same time period.

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Charlie Koven @ckoven.bsky.social · 16h

100 years after they diverge, the zero emissions and CDR pathways reach the same level of cumulative emissions again. This lets us separate the time-evolving response under zero emissions from the response to first positive, and then negative CO2 emissions, using ten Earth system models (ESMs).

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Charlie Koven @ckoven.bsky.social · 16h

We use the flat10MIP experiments to investigate and compare these two sets of changes. In these idealized CO2 emissions-driven experiments, three pathways diverge after 100 years of emissions: they either keep going, abruptly drop to zero, or gradually decline through zero to negative emissions.

this shows traces of lines for three experiments, with cumulative CO2 emissions on the y axis and time on the x axis. one line, labelled 'esm-flat10', is a straight line that goes up and to the right from the origin. the next line, labelled, 'esm-flat10-zec', diverges at year 100 from the black line to be a flat line at a consrtant cumulative CO2 emissions, indicating that annual CO2 emissions have gone to zero. The third line, labelled 'esm-flat10-cdr', also diverges from the same point, but follows a curved trajectory so that it first exceeds the zero-emissions line and then returns to the same level after 100 years. The difference between the two lines from the year of divergence and the year where they reconverge is labelled "Zero Emissions Response" for the flat line, and "Exceedance and Return Response" for the curved line.

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Charlie Koven @ckoven.bsky.social · 16h

New Preprint **Not yet peer-reviewed** How would the climate differ if we exceed a given temperature and then return to it via carbon dioxide removal, versus if we stop emitting CO2 at that temperature and let the climate system evolve over the same length of time?

egusphere.copernicus.org

A Comparison of Regional Climate Changes Under Idealized Zero-Emissions and Exceedance-and-Return Experiments

Abstract. We examine the behavior of ten Earth system models under a set of idealized CO2 emissions-driven simulations to look at regional temperature and precipitation changes in response to either a...

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IPCC @ipcc.bsky.social · 15/09/2026

#NowHiring | Graphic Designer The Technical Support Unit of the #IPCC Working Group I at 📍 Gif-sur-Yvette, France seeks a Graphics Designer to support the development of info graphics, data visualization, & the process of approving the report. 🔗 www.ipcc.ch/about/vacancies

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Charlie Koven @ckoven.bsky.social · 09/09/2026

stop, you’re training the LLMs!

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Charlie Koven @ckoven.bsky.social · 09/09/2026

Gosh, I hate to imagine how expensive their liability insurance must be.

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Charlie Koven @ckoven.bsky.social · 02/09/2026

This is basically my take. It doesn’t really matter whether we think net negative emissions are plausible or not, because we won’t be the ones to make that decision, and the things we need to do now are the same whether net zero is the goal or just one step along a path.

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Charlie Koven @ckoven.bsky.social · 17/07/2026

We've just submitted the First Order Draft of the Working Group 1 contribution to the next IPCC climate report! And registration for reviewing it is open—interested experts, please sign up!

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Paul Voosen @voosen.me · 15/07/2026

Admirable job by what must be @eaterofsun.bsky.social bringing together the ongoing mystery of the surge in absorbed sunlight on Earth. CERES, pattern effects, ECS -- oh my.

economist.com

The rate at which Earth is absorbing energy is alarming climate scientists

Reflections on a warming planet

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Charlie Koven @ckoven.bsky.social · 05/06/2026

This article provides some good local context on the 10-year fight that the city of Oakland has been waging to prevent the new coal export terminal that the Trump administration has just funded from ever being built.

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Alexandra Witze @alexwitze.bsky.social · 08/05/2026

OK, so I was at the first court hearing over the NCAR lawsuit this morning in Denver. I, uh, am not a legal reporter so bear with me as I try to recap a few things. You can view all the court documents here: www.courtlistener.com/docket/72496...

courtlistener.com

University Corporation for Atmospheric Research v. National Science Foundation, 1:26-cv-01061 - CourtListener.com

Docket for University Corporation for Atmospheric Research v. National Science Foundation, 1:26-cv-01061 — Brought to you by Free Law Project, a non-profit dedicated to creating high quality open lega...

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Bob Kopp @bobkopp.net · 25/04/2026

And LAM 2 is a wrap. *So* nice to be working closely w/smart, kind colleagues from around the world toward the common goal of science-informed policy. Lots of work to do between now and July to prepare the first order draft of the physical science report, then to meet again in Arusha in November.

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Claire Zarakas @czarakas.bsky.social · 13/03/2026

Today is the deadline to respond to NSF's Dear Colleague Letter about restructuring NCAR -- there is still time to submit comments by emailing NSF_NCAR@nsf.gov! www.nsf.gov/funding/info...

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Karen Street @karenstreet.bsky.social · 07/03/2026

SF Stand Up For Science, a few of the signs.

Science and Vaccines yes, war and king no

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BART @bart.gov · 19/02/2026

Congrats to BART rider and Oakland legend Alysa Liu on winning a gold medal at the Olympics and making the Bay Area proud!

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Abigail Swann, PhD @ecoclimatelab.bsky.social · 19/02/2026

Someone without a science degree or any science research experience nominated to be in charge of NSF.

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Kata Karáth @kkdfv.bsky.social · 16/01/2026

My latest for @science.org. - Illegal miners have forced Peru’s oldest environmental research station to suspend work. Gold mining has so far led to the deforestation of 140,000 ha of Amazonian rainforest in Peru - about half the size of Yosemite National Park www.science.org/content/arti...

science.org

Illegal miners have forced Peru’s oldest environmental research station to suspend work

Staff at the Panguana biological research station have received death threats

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Cheryl Rofer @cherylrofer.bsky.social · 13/02/2026

It is hard to watch NSF bow so easily to their destroyers. Vannevar Bush formed the NSF out of the US response to the fascists in WWII.

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Ben Sanderson @benmsanderson.bsky.social · 10/02/2026

The federal government is setting itself up to be on the wrong end of the biggest lawsuit in history.

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Dan Visioni @danvisioni.bsky.social · 24/01/2026

We knew the NSF letter about NCAR would come. I think this is an occasion that will require great coordination and strategical thinking. UCAR should respond as a block, coordinating a response across universities (especially in the South, and large public universities that use NCAR data a lot)

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Daniel Swain @weatherwest.bsky.social · 23/01/2026

I would strongly encourage those who value the scientific & societal legacies of NCAR to offer feedback by March 13. I would expect, under present circumstances, perspectives from industry partners will be especially critical, though everyone's voice is of course important.

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AGU Science Policy @aguscipolicy.bsky.social · 09/01/2026

This week, the Trump Administration announced it's decision to withdraw from international organizations and treaties that support global cooperation on climate and environmental issues, a huge setback for global science. Read AGU's full statement: news.agu.org/press-releas...

news.agu.org

AGU statement on US withdrawal from climate and environmental organizations and treaties

The Trump Administration’s announcement that the United States will withdraw from a multitude of international organizations and treaties that support global cooperation on climate and environmental i...

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Charlie Koven @ckoven.bsky.social · 09/01/2026

someone needs to get him on the phone, quick!

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Pam McElwee @pammcelwee.bsky.social · 08/01/2026

The federal govt may be burning global cooperation on climate science to the ground, but we can and are pushing back where we can. We have more than 70 US scientists currently on IPCC reports, with more to be selected this month for CDR methods report. The USAA-IPCC coalition is up for this fight.

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Charlie Koven @ckoven.bsky.social · 08/01/2026

So this is how they’ve chosen to mark the one year anniversary of the Los Angeles fires.

nytimes.com

Trump Pulls Out of Global Climate Treaty

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Ben Bond-Lamberty @benbondlamberty.bsky.social · 05/01/2026

The deadline for DOE's SULI (Science Undergraduate Laboratory Internship) summer 2026 program is in TWO DAYS: science.osti.gov/wdts/suli/Ke... Are you an undergraduate interested in STEM? Apply!

science.osti.gov

SULI Key Dates | U.S. DOE Office of Science (SC)

The page provides the information about SULI Key Dates.

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NWS Bay Area Bot @nwsbayareabot.bsky.social · 02/01/2026

The expected 2.5 ft above normal high tide on Saturday morning hasn't been seen since 1998 x.com/NWSBayArea/status/20070144645…

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Marshall Shepherd @drshepherd2013.bsky.social · 19/12/2025

Please read www.forbes.com/sites/marsha...

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Pam McElwee @pammcelwee.bsky.social · 17/12/2025

Bravo to the #AGU25 protestor out in front of the entrance to the convention center this am

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Charlie Koven @ckoven.bsky.social · 17/12/2025

NCAR’s Mesa Lab is literally a science castle dedicated to understanding our planet in an open and collaborative way, and leading the world in that effort. We cannot let this happen.

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Rosie Fisher @rosieafisher.bsky.social · 17/12/2025

The beautiful idea of NCAR, to improve understanding of our planet through a cooperative, shared and open intellectual team effort, is literally what has got me out of bed every morning for nearly two decades. This cannot, CANNOT be allowed to happen. eu.usatoday.com/story/news/p...

eu.usatoday.com

Trump moves to dismantle major US climate research center in Colorado

The Trump administration is breaking up the National Center for Atmospheric Research, taking aim at one of the world's leading climate research labs.

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Rep. Joe Neguse @neguse.house.gov · 17/12/2025

A deeply dangerous — and blatantly retaliatory action against Colorado — by the Trump administration. NCAR is one of the most renowned scientific facilities in the WORLD — where scientists perform cutting-edge research everyday. We will fight this reckless directive with every legal tool we have.

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Jacquelyn Gill @jacquelyngill.bsky.social · 17/12/2025

They’re stealing from us. They are stealing from us and we have to put a stop to this. This cannot stand.

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Meade Krosby @meadekrosby.bsky.social · 17/12/2025

This wouldn't just be a national tragedy but a global one. Just mind-boggling vandalism.

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Governor Gavin Newsom @governor.ca.gov · 15/11/2025

This 15-story data tower in the heart of the Amazon collects vital climate data that is shared with Americans in California and across the country. The Trump Administration can ignore reality and worsen the climate crisis, but California will never stop fighting for our future.

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Pep Canadell @pepcanadell.bsky.social · 13/11/2025

1/2 We just published the Global Carbon Budget 2025, with a mix of bad news (CO2 emissions continue to grow) and encouraging news (35 countries saw emissions decline over the past decade while growing their economies). Read the highlights in a short article: theconversation.com/the-worlds-c...

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