It depends on who you talk to. The G77 and China, as well as the EU, would suggest that a 50% global emissions reduction means 80% to 90% on the part of developed countries. I'm not sure that's the position of what is commonly referred to as the “umbrella group alliance of countries”, the U.S., Canada, Australia, New Zealand, and others, but for the most part, out of some 170 countries, about 150 would assume that 50% global emissions would eventually mean something in the area of 80% to 90% for OECD countries.
It's a huge challenge; there's no question about it.