As the UN's flagship conference on climate change, COP26, winds down in Glasgow, it is difficult to avoid an overarching impression that this year's summit has been a disappointment. Although not quite over yet (there is a day left at the time of writing), the direction was set early on, and the chance for the most decisive action really departed with the most important presidents, chancellors and prime ministers who attended the opening. After that, even its most enthusiastic proponent, UK Prime Minister Boris Johnson, seemed to lose interest.

The leaders left behind hundreds of negotiators to try and salvage what has been the most high-profile of the annual conferences the UN has been organising for close to three decades now (as the name suggests, it was the 26th edition), ever since the 1992 Earth Summit in Rio de Janeiro. Their plenary sessions have been running into the small hours, full of objections raised by one side or the other, dense technical detail and backpedalling. The aim of the summit was kept simple by the UK presidency: keep 1.5 degrees alive. By that it meant it wanted to gather enough promises to limit global temperature rises to no more than one and a half degrees above pre-industrial levels.

After crunching through the promises made in the first week by nations, financiers and corporations alike, the Independent Energy Agency believes there was enough to hold the rise in global temperatures to 1.8 degrees by the end of the century. That could have been a landmark moment, marking the first time that governments have offered up targets ambitious enough to hold global warming to below 2 °C. But it is too optimistic, working on the presumption that all the promises would have to be met in full and on time, as would all the pledges made previously. No one should, in all seriousness, expect that to happen.

A more realistic calculation by the independent researchers at Climate Action Tracker estimates COP26 pledges could bring the world 9% closer to achieving the 1.5 degree target by 2030. In an analysis, published Thursday, CAT said that sectoral goals on methane, coal, transport and deforestation proposed in Glasgow could cut the equivalent of 2.2 gigatons of carbon dioxide. However, even with all the new climate pledges, global emissions were expected to be "almost twice as high in 2030 as necessary for a 1.5°C compatible pathway."

Meanwhile developing nations are furious that the world's richer countries have already broken a vow, made twelve years ago, to give them $100 billion to help fight climate change. At the moment emissions remain on course to rise by 16% over the next decade. That is a yawning gap between where we need to be and where we are, and so you can understand the scepticism of climate activists like young Greta Thunberg, who didn't have to wait till the end of the summit to lambast the UN-brokered process for turning into a public relations exercise.

"It is not a secret that COP26 is a failure. It should be obvious that we cannot solve the crisis with the same methods that got us into it in the first place," Thunberg said. That is an indictment that invites deep retrospection.