Climate activists have criticized a landmark European Union Green Deal, which aims to reduce greenhouse gases to zero by 2050, for not ramping up its shorter-term goals.

The EU is unveiling its first ever climate law on Wednesday, a legally binding ruling to make the 27-country bloc climate neutral by the mid-point of the century. Greta Thunberg, though, is among campaigners who want to see upgraded 2030 targets as part of the deal.

Thunberg was invited to Wednesday's climate talks by the European Commission president, Ursula von der Leyen.

A group of 34 youth climate activists have signed an open letter, saying the EU should be looking at making shorter-term changes and a "CO2 budget, which applies for today."

The EU has moved to set in stone the 2050 goal, with a leaked document stating that a mechanism will be put in place for regularly raising emission-reduction targets. But there is no plan under the new law to increase the EU's emissions goal for 2030.

The European Commission has said it will review the current goal of a 40 percent greenhouse gas reduction before September this year and look at new targets for 2030. It will "explore options for a new 2030 target of 50 percent to 55 percent emission reductions compared with 1990 levels," the body said.

Greenpeace, the environmental NGO, claims it will now be difficult to have a new 2030 goal ahead of the United Nations climate talks in November. It tweeted: "If your house were on fire, would you wait to 2050 to put it out."

World leaders agreed in 2015 to keep global warming below 2 degrees Celsius and ideally no more than 1.5 degrees Celsius by the end of the century, but scientists are saying those targets will be missed by a wide margin unless drastic steps are now taken.

Greenpeace has dismissed the 55 percent reduction target for 2030 as being insufficient to limit global heating to 2 degrees Celsius, while the World Wide Fund for Nature (WWF) has advised cuts to greenhouse emissions by at least 65 percent by 2030.

The WWF also wants EU leaders to ban subsidies and tax breaks for fossil fuel industries and put in place an independent scientific body to oversee the climate change plans.

The EU's legally binding mechanism to reduce emissions is likely to spark concern from countries that are currently dependent on fossil fuels and will have to reorganize their economies to meet the 2050 target.