{"id":484815,"date":"2026-03-19T23:06:09","date_gmt":"2026-03-19T23:06:09","guid":{"rendered":"https:\/\/www.newsbeep.com\/uk\/484815\/"},"modified":"2026-03-19T23:06:09","modified_gmt":"2026-03-19T23:06:09","slug":"a-field-guide-to-the-runaway-greenhouse","status":"publish","type":"post","link":"https:\/\/www.newsbeep.com\/uk\/484815\/","title":{"rendered":"A field guide to the runaway greenhouse"},"content":{"rendered":"

In a recent post<\/a> on his Substack, Jim Hansen wrote about \u201crunaway climates\u201d on Earth, and I thought it would be useful to explain the physics of what this actually means and whether it\u2019s something we need to worry about.<\/p>\n

A runaway greenhouse occurs when humans add enough carbon dioxide to the atmosphere to push it past a threshold beyond which warming becomes self-sustaining and unstoppable.<\/p>\n

Venus is the canonical example of what that looks like. At some point in the past, it probably looked a lot like Earth. But then the planet heated up and boiled the oceans. Over time, ultraviolet radiation from the Sun split the water molecules into constituent hydrogen and oxygen atoms and the hydrogen escaped to space. In this way, the planet lost its water.<\/p>\n

The loss of the ocean also caused the carbon cycle to shut down. This means that, unlike on Earth, where carbon dioxide is continuously removed from the atmosphere by dissolving into the ocean, there\u2019s nothing removing carbon dioxide from the Venusian atmosphere. As a result, carbon dioxide accumulated in the atmosphere. <\/p>\n

Today, Venus has an atmosphere that\u2019s 90 times more massive than the Earth\u2019s and it\u2019s almost entirely carbon dioxide. The resulting greenhouse effect leads to surface temperatures around 450\u00b0C (850\u00b0F) \u2014 basically the temperature of your oven on self-clean cycle \u2014 and under pressures comparable to those at the bottom of the ocean. <\/p>\n

In this hellish environment, spacecraft engineered specifically to survive Venusian conditions are still only able to last one or two hours before they are destroyed by the harsh environment. <\/p>\n

So let\u2019s take a step back and consider what it takes for the climate to run away.<\/p>\n

If you add carbon dioxide to our atmosphere, you get an initial amount of warming due to increased trapping of heat. If that were all there was to climate change, it would be a pretty simple scientific problem. But lots of things change in response to that warming and some of these changes can lead to additional warming. This is known as a feedback. <\/p>\n

An example is the ice-albedo feedback. As the climate warms, you melt ice. This NASA figure shows sea ice is bright white, meaning that it is reflecting most of the sunlight that hits it. Open ocean is dark blue, meaning that it\u2019s absorbing most of the sunlight that hits it. <\/p>\n

When sea ice melts due to warmer temperatures, a bright white surface (ice) is replaced with a dark surface (ocean). The net result of this change in surface reflectivity is that the Earth absorbs more sunlight. <\/p>\n

This causes more warming, which melts more ice, which leads to more absorbed sunlight and more warming, which drives more ice melt, etc. Lather, rinse, repeat.<\/p>\n

To get more quantitative about feedbacks, let us \ufb01rst go over some basic feedback math. For any feedback, we can express its strength as f, which is the warming produced by one additional trip through this feedback loop per degree of warming from the previous trip through the loop. <\/p>\n

Thus, in response to an initial warming \u2206Ti, the \ufb01rst trip through the feedback loop produces additional warming of f\u2206Ti due to melting ice and increased absorption of sunlight. <\/p>\n

But the feedback operates on this additional warming, producing an additional warming of f \u00d7 f\u2206Ti = f2\u2206Ti. The feedback then operates on this additional warming too, leading to an additional warming of f \u00d7 f2\u2206Ti = f3\u2206Ti . This goes on forever, so the \ufb01nal warming \u2206Tf is:<\/p>\n

\\(\\Delta T_f = \\Delta T_i + f \\Delta T_i + f^2 \\Delta T_i + f^3 \\Delta T_i + \\cdots \\)<\/p>\n

The math ninjas among you will recognize this infinite series can be rewritten simply as:<\/p>\n

\\(\\Delta T_f= \\frac{\\Delta T_i}{1 – f}\\)<\/p>\n

As long as f remains below 1, each pass through the feedback loop contributes less warming than the last, and the infinite series converges to a finite total. But when f reaches 1, every pass contributes the same warming as the one before \u2014 the series never converges, and the total warming becomes unbounded. This is a runaway climate.<\/p>\n

The temperature won\u2019t actually reach infinity, of course. Every feedback has a physical limit at which it shuts off. The ice-albedo feedback, for instance, stops amplifying warming once all ice has melted. The water vapor feedback stops once the oceans have boiled away and no liquid water remains. These are natural endpoints that have all occurred on Venus.<\/p>\n

But the essential question stands: can warming on Earth ever take off on its own? Can Earth reach a runaway climate?<\/p>\n

We know our current climate is stable \u2014 we add warming every year, and the climate hasn\u2019t run away. This tells us that Earth\u2019s f is well below 1. Detailed analyses of all the major feedbacks \u2014 water vapor, clouds, ice albedo, and the lapse rate<\/a> \u2014 confirm this, placing the combined f somewhere between 0.5 and 0.75. A stable climate, just as we\u2019d expect.<\/p>\n

Share<\/a><\/p>\n

There are a few ways that have been proposed whereby the Earth\u2019s atmosphere runs away. They mostly revolve around the behavior of water vapor in the atmosphere.<\/p>\n

warning: you are entering the nerd zone<\/p>\n

If the surface warms enough, the atmospheric temperature profile asymptotes to the saturation vapor pressure curve, known as a pseudoadiabat. In this state, the temperature at a given optical depth becomes fixed, regardless of the surface temperature. <\/p>\n

Consequently, the energy out for the Earth reaches an asymptotic upper bound, known as the Simpson-Nakajima limit. If energy in for the Earth exceeds this maximum energy out, the energy imbalance cannot be corrected by a rise in surface temperature, leading to a runaway greenhouse effect.<\/p>\n

People have estimated the limit of energy in to be around 280-290 W\/m2. Right now, energy in from the Sun is about 240 W\/m2, so this suggests that it would take at least +40 W\/m2 of radiative forcing to push Earth into a runaway climate state. Given that doubled carbon dioxide has a radiative forcing of +4 W\/m2, +40 W\/m2 corresponds to 10 doublings of carbon dioxide, a factor of 1000x above preindustrial concentrations.<\/p>\n

It seems quite unlikely that there is anywhere near enough fossil fuels to reach this runaway threshold. <\/p>\n

A far more plausible way for our climate to run away is a brightening Sun. Solar physicists think that the Sun is getting brighter at a rate of about 1% per 100,000,000 years. At this rate, energy in for the planet might reach the runaway threshold in one or two billion years. <\/p>\n

Not about Venus. A true runaway greenhouse \u2014 boiling oceans, hellish surface temperatures \u2014 is simply not in the cards on any time horizon we care about.<\/p>\n

But \u201cnot Venus\u201d is a remarkably low bar to set for ourselves. There\u2019s an enormous amount of territory between our current climate and a Venusian apocalypse, and we\u2019d strongly prefer not to explore it. As Jim Hansen has argued, what we should really care about is warming sufficient to break the things that matter to us \u2014 ecosystems, infrastructure, food and water security, human lives.<\/p>\n

We\u2019re already seeing bad things happen<\/a> to those \u2014 with only 1.5\u00b0C of warming. Climate change is affecting us right now, without any runaway greenhouse in sight. That alone should be more than enough motivation to limit future warming as much as we can.<\/p>\n

Another possible scenario for very large warming is the emergence of a strong carbon-cycle feedback: A truly worst-case climate scenario: Losing control of the carbon cycle<\/a><\/p>\n

And, of course, clouds could do something weird<\/a>.<\/p>\n

Ray Pierrehumbert\u2019s excellent book Principles of Planetary Climate<\/a> contains several illuminating discussions of a runaway greenhouse (note: this book is at the grad school level).<\/p>\n

\"\"<\/a><\/p>\n

I wrote recently<\/a> about the cost of fossil fuels to our national and economic security. Then came reports that the Pentagon has asked for approval for a $200 billion request to Congress to pay for the war in Iran. This is a lot of money \u2014 around $600 per U.S. resident. <\/p>\n

This war, as well as the war in the Ukraine, are fundamentally enabled by fossil fuels. In a world of climate-safe renewable energy, it\u2019s hard to see that these wars are being waged. <\/p>\n

This really undercuts the \u201csolar and wind are expensive\u201d argument. <\/p>\n

I\u2019d be grateful if you could hit the like button \u2764\ufe0f below! It helps more people discover these ideas and lets me know what\u2019s connecting with readers.<\/p>\n","protected":false},"excerpt":{"rendered":"In a recent post on his Substack, Jim Hansen wrote about \u201crunaway climates\u201d on Earth, and I thought…\n","protected":false},"author":2,"featured_media":484816,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[22],"tags":[1397,90,56,54,55],"class_list":["post-484815","post","type-post","status-publish","format-standard","has-post-thumbnail","category-environment","tag-environment","tag-science","tag-uk","tag-united-kingdom","tag-unitedkingdom"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/484815","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/comments?post=484815"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/484815\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media\/484816"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media?parent=484815"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/categories?post=484815"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/tags?post=484815"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}