Global Warming is Only Part Human Caused

So how much global warming is human caused?

It's a good question and as with anything in complex science needs context to be well understood. If we look at CO2 only for example we can easily be mislead.

Here's why:

Pre-industrial CO2 for our current interglacial was around 280ppm. That's just under 3 hundredths of a percent. Now it is at 387ppm (2008 measurement). That's around 38%.

Does that mean that global warming is only 38% human caused and the rest is natural cycle? No.

Milankovitch Cycles

The Milankovitch cycles are generally passed what is called perihelion. That means they are past peak warming influence in the natural cycle. That means we are supposed to be cooling.

Natural Cycle

The natural cycle is typically considered the long term variations that affect climate such as the Milankovitch Cycles. In the natural cycle we should be cooling. In fact, we were cooling just prior to the industrial revolution. But now we are warming. So let's review, we were cooling in the natural cycle and now we are warming.

This warming is quite clearly attributable to the imposition of industrial greenhouse gases introduced to our atmosphere.

Natural Variation

Natural variation happens whether climate change is human caused or not. The natural variability is caused by the dynamic nature of interactions between the ocean cycles and seasonal, decadal or even longer term changes to land or ocean systems. Generally though, natural variation is the normal ups and downs of temperatures in the intra-decadal and inter decadal. As always you need to know the context involved.

Virtually 100% Human Caused

It is safe to say that virtually 100% of this warming event is human induced, caused, or instigated by human industrial processes and output of industrial GHG's.

100% Human Caused

Another way to reasonably view the percentage of cause is the path of warming, or trend. When one examines the new path, one can plainly see that it is 100% human caused. Humans have altered the path of the climate of earth.

Summary

We can reasonable see that the natural system is still functioning but that it was near neutral and likely biased slightly to cooling. Industrial based GHG's have increased the forcing levels and altered the climate path. That would indicate that the new path is reasonably described as 100% human caused.

Another way to illustrate the percentage of human cause would be to consider the course of climate pre-industrial (cooling) and add the warming. In other words climate  had to reverse its course so one might then argue this warming event is even greater than 100% human caused.

Links

• 2005 - June: How much of the recent CO₂ increase is due to human activities?