In a new book, the Swedish activist said the use of “clever accounting” and legal loopholes told a misleading story of progress.
She said Britain’s claim to climate leadership was undermined by its oil and gas extraction and imports from carbon-heavy nations.
On the eve of the summit, Ms Thunberg’s new work The Climate Book brings together more than 100 experts and authors to set out the scale of the task.
In it, she takes the view that it is now too late to save the climate without major lifestyle changes.
She despairs at leaders who ask her basic questions and is pessimistic that major polluters are making progress.
Her own country, Sweden, has barely cut emissions at all once those from aviation and shipping are included, she says.
These sectors are known as “international bunkers” that do not count towards an individual country’s emissions.
But aviation emissions in the European Union rose by more than 120 per cent between 1990 and 2017, according to official figures.
The EU would “never stand a chance of reaching its climate targets without a wide use of clever accounting,” Ms Thunberg says.
In Britain, meanwhile, campaigners such as Ms Thunberg say the Drax power station in Yorkshire is the country’s biggest CO2 emitter.
But many of these emissions are not counted because the plant runs on biomass, such as wood, which is treated as carbon neutral.
This is because trees absorb CO2 from the atmosphere before it is re-emitted at a power plant ― but many activists see this as greenwashing.
"Since trees grow back, we have decided that cutting them down and shipping them halfway around the world in order to burn them is considered renewable," Ms Thunberg writes.
In addition, some of Britain’s emissions have been outsourced to other countries in the form of imports, she says.
She says the only good news in Britain is emissions cuts in the power sector, where the use of coal has plunged since 1990.
Greta Thunberg through the years - in pictures
However, an official figure of 43 per cent emissions cuts between 1990 and 2018 is too generous and should be calculated at about 23 per cent, she says.
“What we are actually being told to celebrate ― over and over again ― is outsourcing, excluding emissions, clever accounting and the negotiation of global frameworks that make it all perfectly legit,” Ms Thunberg writes.
“Or, more to the point, we are being told to celebrate the fact that we are cheating.”
The EU is aiming for a 55 per cut in emissions by 2030, compared with 1990 levels. The UK is aiming for 78 per cent by 2035.
EU diplomats reached a deal on Thursday to make zero-emission engines compulsory for new cars by 2035.
In her book, Ms Thunberg calls for public transport fares to be abolished and suggests people should give up what she calls the privilege of flying.
But a theme she returns to several times is that the action she regards as most effective is informing other people how urgent the crisis is.
Ms Thunberg first rose to fame with her weekly school strikes to raise awareness about climate change.
She writes that she did not sail across the Atlantic Ocean to shame people or because it would put a major dent in carbon emissions.
Her aim was to signal to people that climate change is an emergency and that major lifestyle changes are needed, she says.
Despite her pessimism about emissions cuts, she ends the book with the optimistic belief that action will be taken if people believe it is necessary.
She cites the response to the coronavirus pandemic and the war in Ukraine as proof that drastic action can be taken if policymakers treat a crisis as urgent.
“Humans are not evil, and once we understand the nature of the crisis we will surely act. Given the right circumstances, there are no limits to what we can do,” she writes.
“Once we have been given the full story ― and not something that has been conjured up to benefit certain short-term economic interests ― we will know what to do. There is still time to undo our mistakes, to step back from the edge of the cliff and choose a new path, a sustainable path, a just path.”