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 misled. The answer to the question on the context of the question. In other words Are we asking how much of total warming on earth is human caused, or are we asking what percentage of the recent changes can be attributed to human cause?
Pre-industrial CO2 for our current interglacial was around 280ppm. That's just under 3 hundredths of a percent of the atmospheric composition. Now it is at 399ppm (Dec-2014 measurement (see current on CO2 monitor page). That's about a 42% increase from pre-industrial.
Does that mean that global warming is only 42% human caused and the rest is natural cycle? No. But even that is too simple an answer. You have to look at the big picture and then separate the little pictures.
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.
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 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 certainty regarding cause is the path of warming, or trend. When one examines the new path, one can plainly see that it is unequivocally human caused. This subject is being looked at by thousands of scientists. If there really was a way to prove that the current change in path of the climate was not human caused, someone would have at least found something and made the claim in the peer reviewed literature, and if it actually was substantial, it would have made world headlines in the scientific and public media simultaneously.
Humans have clearly altered the path of the climate of earth. Scientifically one never claims 100% certainty. However, one can say the conclusions are unequivocal. This essentially means that there are no other theories that can invalidate the conclusion.
We can reasonably see that the natural system is still functioning, but that it was near neutral (thermal equilibrium) and possibly 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 simply 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 the forcing required to achieve current warming is even greater than 100% human caused. But this sort of view needs context. First you have to stop the cooling with a forcing. Then you have to have sufficient forcing to continue in the opposite direction.