How much of the current global warming is due to human influence?

Question

Approximately what proportion of the global warming seen over the the last century is attributed to anthropogenic sources?

Answer 1

Firstly it is worth demonstrating that 97% of climate scientists agree that climate-warming trends over the past century are very likely due to human activities.

W. R. L. Anderegg, “Expert Credibility in Climate Change,” Proceedings of the National Academy of Sciences Vol. 107 No. 27, 12107-12109 (21 June 2010); DOI: 10.1073/pnas.1003187107.

P. T. Doran & M. K. Zimmerman, "Examining the Scientific Consensus on Climate Change," Eos Transactions American Geophysical Union Vol. 90 Issue 3 (2009), 22; DOI: 10.1029/2009EO030002.

N. Oreskes, “Beyond the Ivory Tower: The Scientific Consensus on Climate Change,” Science Vol. 306 no. 5702, p. 1686 (3 December 2004); DOI: 10.1126/science.1103618.

But what percentage of the increase is attributed to humans? Probably almost of all of it.

The scientist Gavin Schmidt from NASA was asked this question on realclimate.org

His response was as follows:

Over the last 40 or so years, natural drivers would have caused cooling, and so the warming there has been (and some) is caused by a combination of human drivers and some degree of internal variability. I would judge the maximum amplitude of the internal variability to be roughly 0.1 deg C over that time period, and so given the warming of ~0.5 deg C, I'd say somewhere between 80 to 120% of the warming. Slightly larger range if you want a large range for the internal stuff. [emphasis added]

The rapid increase in the human-driven component of the forcing are increasingly dwarfing the small, slow natural forcings, rendering them increasingly irrelevant.

Answer 2

From the IPCC Fifth Assessment Report (2014 / current):

It is extremely likely that human activities caused more than half of the observed increase in global average surface temperature from 1951 to 2010. This assessment is supported by robust evidence from multiple studies using different methods. In particular, the temperature trend attributable to all anthropogenic forcings combined can be more closely constrained in multi-signal detection and attribution analyses. Uncertainties in forcings and in climate models’ responses to those forcings, together with difficulty in distinguishing the patterns of temperature response due to WMGHGs and other anthropogenic forcings, prevent as precise a quantification of the temperature changes attributable to WMGHGs and other anthropogenic forcings individually. Consistent with AR4, it is assessed that more than half of the observed increase in global average surface temperature from 1951 to 2010 is very likely due to the observed anthropogenic increase in WMGHG concentrations. WMGHGs contributed a global mean surface warming likely to be between 0.5°C and 1.3°C over the period between 1951 and 2010, with the contributions from other anthropogenic forcings likely to be between –0.6°C and 0.1°C and from natural forcings likely to be between –0.1°C and 0.1°C. Together these assessed contributions are consistent with the observed warming of approximately 0.6°C over this period (Figure TS.10). {10.3}

Observed: temperature measurements

WMGHG or GHG: Well-mixed greenhouse gases like CO2

ANT: Sum of GHG and OA

OA: Other anthropogenic forcings on temperature like aerosols and black carbon

NAT: Natural variability such as change in solar radiation Internal Variability: Estimated lack of knowledge in current earth models