Why do carbon emissions matter?

This is the question addressed in Lord Nicholas Stern’s first book since the highly influential Stern Review. It sounds like a rather simple question but one that is also very important to get right. It is, according to Stern, the requisite for a global deal on climate change. So, after hearing his short speech on the book I’ll try to quickly give a recap.

1) Carbon emissions arise from everything that humans, and animals do. But greater volumes of production and consumption, or destroying natural emission “sinks” (these are trees and plants that actually absorb carbon emissions) create greater and greater flows of emissions into the atmosphere.

2) The atmosphere absorbs some of these emissions, but only a fixed amount. So as the excess emissions are released, they build up and create a “stock” of emissions in the atmosphere (measured in parts per million).

3) This stock of emissions influences the global average temperature. Right now there are 380 parts per million of emissions in the atmosphere and growing and a temperature increase of close to 2 degrees Celsius. As this number increases to 450, 500, 600 and beyond, the global average temperature also increases.

4) This global average temperature change results in extraordinary consequences for the earth’s climate. A 1 or 2 degree change produces noticeable consequences in water, ecosystems, and agriculture. A 4 or 5 degree change potentially (probably) produces an ice-age.

5) These climate changes have profound affects on the way that we live our lives. The figure below, from the original Stern Review, documents these changes. Small climate changes alter Cities, human movements, food sources, disease patterns, and more costly but potentially acceptable consequences. But large climate changes, such as a new ice age produce unimaginable human consequences. Imagine the world’s population in 2100 trying to fit into a small strip of habitable land around the earth’s equator.

Thinking clearly about this chain of events allows us to answer clearly and definitively how and why carbon emissions matter. It also allows people like Stern to do the arithmetic to calculate exactly how few carbon emissions we need to produce to ward off the extraordinarily devastating consequences. It is, according to him, 2 tons of emission per person per year by 2050. But now that we’re clear on why and how emissions matter, we (you) can debate exactly what this number should be. Next we can debate how best to get there.