The point that they are trying to make is very simple:
Our planet cannot sustain unlimited growth.
I know, it should be obvious. Sadly, though, some economists still seem to miss this incredibly important point:
"It is precisely this economic growth which will lift the poor out of poverty and improve the environmental standards that really matter to people - like clean air and water - in the process, as it has done throughout human history."
That is Tom Clougherty from the Adam Smith Institute, which apparently operates "Europe's favourite (denialist) think tank website," responding to the NEF study in a BBC article.
The NEF goes beyond making a simple point with a very cute monster hamster in the meat of their study:
Even at a growth rate of 3 per cent (low for many developing countries), the global economy would need to reduce its carbon intensity by 71 per cent by 2050 (compared to 2002) or 2.7 per cent per year. This would mean achieving more than double the yearly average improvement between 1965 and 2002. But even this would result in a level of carbon dioxide (CO2) in the atmosphere of 500 parts per million (ppm). Whereas the latest climate science shows that such a level would push temperature rises far passed the 2 °C threshold.
The point being that if global economic growth continues, it will take much more dramatic reductions in CO2 emissions than we are currently considering, or may even be possible, to prevent disastrous climate change. And climate change is only one of many reasons that we can not continue growing the global economy indefinitely. Earth's ecosystems as they exist today will not last forever if we continue on this unsustainable path.
As I am not an expert, I cannot vouch for the numbers in this study. However, it is difficult to argue against the main point:
Andrew Simms, co-author of the report and nef policy director said, "We tend to think of growth as natural for economies, forgetting that in nature things grow only until maturity and then develop in other ways. A world in which everything grew indefinitely would be strange indeed. A young hamster, for example, doubles its weight each week between birth and puberty. But if it grew at the same rate until its first birthday, we’d be looking at a nine billion tonne hamster, which ate more than a year’s worth of world maize production every day. There are good reasons why things don’t grow indefinitely. As things are in nature, so sooner or later, they must be in the economy.
"The economic priorities of the rich world are as ridiculous as the impossible hamster. Endless growth is pushing the planet’s biosphere beyond its safe limits. The price is seen in compromised world food security, climatic upheaval, economic instability and threats to social welfare. We urgently need to change our economy to live within its environmental budget. There is no global, environmental central bank to bail us out if we become ecologically bankrupt."