From Watts Up With That?

By Charles Rotter

The rainfall data behind it shows no drying trend. The geology behind it has been stable since before the Romans. And the number the press ran was the one from the dead scenario.

Charles Rotter

“Millions of homes in London, Essex and Kent at risk of sinking as climate crisis worsens,” said the Guardian. AOL ran it. The story bounced around the usual outlets, all of them leading with the same number: more than four million British homes at risk from climate-driven subsidence by 2070. The source is the British Geological Survey, which is a serious institution, or used to be, and that is what makes this one worth a few minutes.

Paul Homewood already did the legwork on the rainfall trends, and I will not repeat all of it. He pulled the Met Office’s own spring rainfall series back to 1840 and the summer rainfall for South East England back to 1875, and the short version is that there is no trend. Dry springs were more common a century ago. The last really dry summer in the South East was 1995. If the South of England is drying out, the South of England has not been told. Go read his piece for the charts. They are the kind of charts that end an argument.

What I want to walk through is the foundation the four million number sits on, because once you see it, you cannot unsee it, and it is the same foundation holding up half of British climate policy.

The BGS figure comes from a product called the GeoClimate Shrink-Swell dataset. Shrink-swell is a real thing. Clay soils absorb water and swell, then dry out and shrink, and the ground moves, and houses built on clay crack. London sits on London Clay, which is about as shrink-swell as clay gets. This has been true since before there was a London. It is the reason the Victorians had subsidence and the Edwardians had subsidence and your great-grandmother’s terrace in Camden has a crack over the front door. Subsidence is made worse by the weight of buildings, by water extraction, and by the large trees people insist on planting next to their houses. None of that is new, and none of it is climate.

To turn an old geological fact into a 2070 headline, you need a forecast. And to get the forecast, the BGS took its geological maps and combined them with climate projections. Specifically, per the BGS’s own announcement, climate data derived from the Met Office’s UKCP18 projections.

And there it is. UKCP18.

UKCP18 is the Met Office’s 2018 set of UK climate projections, and its headline-grabbing high-end scenario was built around RCP8.5. Regular readers know where this is going, because we have covered it at length. RCP8.5 is the emissions pathway that assumes a fivefold expansion of global coal use through 2100, no meaningful climate policy, and technological stagnation, none of which is happening or has happened. As of the new ScenarioMIP framework for CMIP7, published this April, RCP8.5 and its successor SSP5-8.5 have been formally retired from the scenarios that feed the IPCC’s next assessment. The climate community itself has stopped using it. We have run several pieces on this in the last few weeks.

So here is the actual structure of the four million homes story. The BGS took its geology, fed it through the Met Office’s UKCP18, which is built on RCP8.5, which the IPCC retired three months ago as implausible, and produced a forecast for 2070.

If this were a single overcooked press release I would not bother. What makes it worth writing about is that it is now the house style of British climate institutions, and it arrives the same week as a related example worth holding up next to it.

Consider sea level, which is the other great engine of British coastal doomcasting. The accelerating-sea-level story rests almost entirely on the satellite altimetry record, which began in 1993 and now runs about thirty years, stitched together across several satellite missions, each handoff requiring a calibration adjustment. The long tide gauges, some running well over a century, tell a quieter story. The Battery gauge in New York has measured a steady rise of roughly 2.8 to 3 millimetres a year since 1856 with no statistically significant acceleration across more than 160 years. Where the short satellite record and the long tide-gauge record diverge in character, there is a real and live argument about which one deserves the greater weight, and it is not the settled question the headlines imply. David Burton has been documenting the tide-gauge side of this at sealevel.info for years, patiently, gauge by gauge.

The two stories rhyme. In both cases a long, boring, directly observed record shows a modest, roughly linear trend that has been going on for as long as we have measured it. In both cases a shorter, model-flavoured construction is used to project an alarming acceleration into the future. And in both cases the alarming version is the one that reaches the public, gets attached to a policy, and becomes the thing “everyone knows.”

What this is for

The BGS announcement is candid about what the dataset is for. It is designed, in its own words, to help local authorities, developers, planners, mortgage lenders and insurers assess their exposure to climate-related risk. In other words, it is intended to flow into planning rules, lending decisions, insurance premiums, and house prices. A forecast built on a retired emissions scenario is going to be used to decide whether your bank will lend against a terrace in Barnet.

And that is the part that should bother people who do not care one way or the other about climate politics. UKCP18 has not been withdrawn. The Met Office still serves it across its web pages. The BGS has now built a property-risk product on top of it. The scenario underneath has been declared implausible by the body that invented it, and the British institutional response has been to carry on as though nothing happened, because the projections are load-bearing. They hold up the Net Zero policy architecture, and you cannot pull the bottom scenario out without the upper floors getting nervous.

So the British Geological Survey, an organisation with a genuinely distinguished history of mapping the actual rocks under actual Britain, has published a forecast of four million sinking homes built on a climate scenario that the climate community retired in April. The rainfall data behind it shows no drying trend. The geology behind it has been stable since before the Romans. And the number the press ran was the one from the dead scenario.

If a graduate student turned this in, the supervisor would hand it back. But it was not turned in by a graduate student. It was published by the BGS, printed by the Guardian, and is now, somewhere in Whitehall, being quietly folded into the case for a policy that was decided long before the homes were counted.