European Union leaders have set a new target landmark deal to cut carbon emissions by 40 percent by 2030.

In a tweet, Herman Van Rompuy, chair of the European Council, the gathering of EU leaders, tagged the deal as the "world's most ambitious, cost-effective, fair climate energy policy agreed."

The EU agreement also includes 27 percent targets for the amount of renewable energy in 2030 and for energy efficiency advancements.

Van Rompuy added that this "is good news for climate, citizen's health, international climate talks, sustainable jobs, energy security and competitiveness. "

But key aspects of the deal that will form a negotiating position for global climate talks next year in Paris were left vague or voluntary. This raised questions as to how the aims would be realized.

European Commission president, Jose Manuel Barroso added, "This package is very good news for our fight against climate change."

Looking forward to the global climate summit in Paris, France next year, EU's climate commissioner, Connie Hedegaard said that this deal have sent "a strong signal to other big economies and all other countries...now we urge you to follow Europe's example.

But a clause inserted to the text of the deal could trigger a point of review for EU's new targets if the other countries don't move forward with comparable commitment in Paris.

Arguments over energy savings and climate policy dominated the Brussels summit. Countries like Portugal and Poland were pleading for special circumstances and threat to reject any plans or breakthrough unless their demands were met.

Poland, heavily dependent on coal-operated energy production, fights for its coal industry and threatened to block any development from this deal unless the costs to its economy and industry were met.

However, the existing goal of a 20 percent cut by 2020 has been nearly met to date.