Just when you thought it couldn’t get worse

When estimating how hot it will be in the future, two main variables are at play: 1) the amount of greenhouse gases trapping energy, and 2) how sensitive the Earth’s climate is to that trapped energy. These combined factors help scientists estimate temperatures for various emissions scenarios, such as those used by the IPCC.

Climate models predict different amounts of warming for the same rise in greenhouse gases. To narrow down which models are right, scientists look at real-world data.

New research by Gergana Gyuleva, Erich Fischer, Reto Knutti (all from ETH Zurich), and Sebastian Sippel (Leipzig University) suggest previous sensitivity estimates were too low.

It turns out that natural climate noise was suppressing surface temperature trends causing previous calculations to underestimate climate sensitivity.

Recent evidence from trends in the short-wave and long-wave components of Earth's energy imbalance (EEI) at the top-of-atmosphere suggests that models with higher TCR lie closer to the observed EEI trends. Here, we reconcile this discrepancy and provide a revised range for TCR of 1.9–2.6 K.

Satellites measuring Earth’s Energy Imbalance (EEI) (the gap between incoming solar radiation and outgoing heat) show that heat accumulation between 2001 and 2025 matches high-warming models.

The planet is tracking the extreme end of the sensitivity to atmospheric CO2.

So what does this mean? The median climate model used to estimate future temperatures has underestimated future global heating.

For example, following a middle-of-the-road trajectory (SSP2-4.5), high-sensitivity models yield end-of-century warming closer to 3.5°C rather than the 2.7°C median. This scenario is based on emissions soon plateauing before declining after 2050. So far, this scenario seems implausible as emissions keep rising and the world remains trapped in an energy-dependent “grow or perish” (ironic, right?) mindset.

Under the more likely heavy emissions pathways (SSP3-7.0 and SSP5-8.5) where emissions double by 2100, high-sensitivity projections exceed 4.0°C to 5.0°C by end of century. Crossing 2.0°C of warming occurs in the 2030s to early 2040s across all moderate-to-high emission pathways under higher sensitivity constraints. This is the path we’re currently on, and this research just informed us we’re tracking the worst case scenario.

Small adjustments, big impacts. Let’s look at what these temperature increases mean.

At 1.5 Celsius (basically where we are today), we’ll see (we already are seeing) increased volatility in crop yields with potential mid-to-high single digit declines for certain staples, such as wheat. (See current crop yield declines in Europe and droughts across UK and US.) Water shortages worsen, affecting urban and rural areas alike. Periodic heat domes punish citizens, and food price shocks push millions more into poverty. The UK just warned citizens to start stocking up on tinned food.

At 2 degrees, expect multiple simultaneous crop failures across the world, with yields declining by double-digits. Breadth and depth of droughts hit half a billion people regularly. Famine and migration become recurring themes. Losses from increased infrastructure damage and reduced worker productivity hit supply chains, punishing the economy and markets.

3 degrees causes chronic food deficits triggered by repeating crop failures and drought. Standard agricultural adaptation methods for major staples fail. Expect widespread supply chain disruptions and failing insurance markets, possibly triggering financial collapse. Governments will print money in response, likely pushing up inflation as physical shortages cannot be compensated by digits in a bank account. Death and displacement is widespread throughout the world. Religion resurges as the hopeless follow anyone holding a candle in the dark.

At 4 degrees the end is nigh. Massive food shortages have already killed billions. Whatever is produced (now agricultural shortages are exacerbated by labor shortages and decrepit logistics) is hoarded by those with the physical power to do so. The planet shifts into reverse as the biosphere cascades uncontrollably into chaos. For those who manage somehow to survive, living standards return to pre-industrial levels. Most live a hand-to-mouth existence, entirely dependent on their own skills for survival. Life becomes a balance between meager rations, roving gangs, and infectious disease. Electricity: fails. Healthcare: gone. Security: abandoned. If you somehow make it this far, you’re on your own.

This is where we’re headed, faster than expected. YMMV.

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