Sir Keir’s speech, expected on Tuesday, coincides with a record “dunkelflaute” spell of low winds and sunshine that have already slashed output from renewables.
Wind speeds are expected to fall again this week, making gas more essential than ever to keep the lights on.
“Many of our weather stations have recorded zero sunshine so far this month,” said a Met Office forecaster. It’s very unusual.”
Those weather stations are spread across the southern UK, with zero sunshine readings from Brize Norton in Oxfordshire, Ross on Wye, near the Welsh borders, St Athan, west of Cardiff, Liscombe in Devon and Wittering, near Peterborough.
The Met Office said: “Monday and Tuesday will see unusually light winds. There’s an area of high pressure across the UK and Europe which makes for very settled and calm weather.”
Weather readings and forecasts have become increasingly important for UK power generation because of growing reliance on wind and solar.
Over the last week solar generated just 0.7pc of the nation’s electricity and wind just 10.6pc, according to grid data.
By contrast, on sunny and windy days renewables have generated more than 87pc of UK power needs.
Neso, the UK’s National Energy System Operator, which runs the grid, expects the UK to have its first periods of complete decarbonisation, when no gas is needed, in 2025.
However, such data also underscore the problem of intermittency, with the grid needing some means of supplying low carbon power all the time, not just when the wind is blowing.
Mr Miliband said the UK’s willingness to pledge such massive cuts in greenhouse gas emissions was a sign of “climate leadership.”
He said: “The only way to protect our children and future generations is by leading global climate action. At the Cop29 climate talks, we will work with other countries to step up ambition on tackling the climate crisis.”