An agreement between countries around the world that could slow down global warming came into force on Wednesday.
The Kyoto Protocol was created seven years ago, but until recently not enough countries agreed with it.
But last year Russia signed up, joining other countries including the UK and Japan that have pledged to cut back on gases that make global warming worse.
However, some people aren't happy that countries like the US and Australia aren't signed up at the moment.
The US is one of the countries that hasn't signed up
The US - where a quarter of the world's carbon dioxide comes from - and Australia haven't signed up because their governments think doing so would hurt their economy.
Others like China and India, have signed up, but aren't completely involved yet because they are still developing countries.
For the agreement to come into force two things had to happen:
- 55 countries had to sign up to it
- Certain countries responsible for producing at least 55% of the gases that make global warming worse also had to sign up to it
Until Russia joined in 2004 those conditions hadn't been met.
Under Kyoto the countries involved have pledged to cut the amount of greenhouse gases they produce by 5.2% by 2012.