The world is heading for an “irrecoverable overshoot” of global temperature thresholds that will plunge billions of people into disastrous climate impacts because governments have stalled for so long to cut planet-heating emissions, analysis has found.

Under current policies, the planet will heat up by 2.7C on average by the end of the century compared with preindustrial times, the report by Climate Action Tracker (CAT) found. This is up slightly from a prediction of 2.6C in the group’s analysis conducted last year.

This level of warming smashes the 1.5C threshold that was agreed by countries a decade ago but is set to be breached in just a few years. Scientists have warned that beyond this point already harmful storms, droughts, heatwaves, flooding and other impacts will become intolerable for parts of the world and potentially trigger runaway climate change that won’t be contained.

Countries, minus the US, are gathered in the Turkish city of Antalya for UN climate talks that have centred on the looming “overshoot” of the temperature target. This reality should compel governments to act quickly to phase out use of the fossil fuels that are causing the climate crisis, the UN secretarygeneral, António Guterres, said on Tuesday.

If countries did act decisively to slash emissions then the future temperature projections would become less daunting, the CAT report states. However, in even the most optimistic scenario – where all pledges, targets and other long-term promises made by countries were fulfilled – the world would still heat up by 1.9C by the end of the century.

Such targets have been routinely flouted by the countries that have made them, however. This means leaders face a crunch moment, experts say.

“The stark choice for governments is to either continue to support the fossil fuel industry, which would consign humanity to insurmountable economic damages, rising mortality from heat and climate-driven diseases and disasters that they are unable to adapt to – or pivot to the solutions which are available and cheaper than ever,” says Bill Hare, chief executive of CAT partner organisation Climate Analytics.

Hare said exceeding 1.5C warming “cannot be an excuse to open the door to defeatism” and that every small increment of temperature rise will be significant. The CAT analysis is similar to one recently put out by the UN, which uses previous CAT data to inform its own projections.

While the world is failing to cut emissions in line with the science and countries’ own promises, carbon pollution is set to dip slightly this year and CAT pointed to some causes for optimism. Clean energy investment is now double that of fossil fuels, electric car use is soaring and the deforestation of the Amazon in Brazil has fallen sharply.

But much more will need to be done to avert disastrous climate impacts. Greta Thunberg, the Swedish climate advocate, arrived in Antalya on Tuesday and warned that “everything and everyone we love is at stake” because of the failure to confront the climate crisis.

“Governments collectively are simply not moving fast enough to adopt the policies that would enable us to escape from the emerging overshoot trap,” said Sarah Heck, lead author of the CAT report. Some countries, such as the US, Canada and New Zealand, are even slowing down their climate action to a “staggering level”, she added.