The graphic is from “The energy balance over land and oceans: an assessment based on direct observations and CMIP5 climate models” (Martin Wild et al, 2015). You can find the paper here.
The paper discusses the progress in understanding overall energy flows between the surface of the earth and the top of the atmosphere (TOA) that are the motor of the earth’s climate. It considers the extent to which the climate models that are the basis for the findings of the latest, 5th, IPCC Assessment Report (AR5), match up with observations. In brief, the paper shows that most of the models significantly overestimate the warming contribution of the sun at the surface of the earth (Fig 7 and the associated discussion).
The graphic shows the IPCCs model of energy flows at the start of the 21st century based on observations by satellites at the top of the atmosphere and at the surface of the earth, including the oceans, by a network of sensors. Notice that the net incoming energy flows (yellow) and the outgoing flow (orange) nearly balance at about 240 watts per square meter (w/m2 ). The difference in the inward and outward flows is illustrated by a small green arrow in the bottom-left of the graphic showing an “imbalance” of about 0.6 w/m2. This imbalance — which the IPCC says is due to the ‘greenhouse’ effect of CO2 (plus various unspecified ‘feedbacks’) — is responsible for global warming. In their account, the feedback-augmented greenhouse effect prevents the full re-radiation of energy to space and the difference is incrementally warming the surface of the earth including the oceans.
What struck me about this graphic is the size of the uncertainty ranges in the measurement of the energy flows. Wild et. al. report that the random error in the observed value of downwelling radiation at the surface of the earth is of the order of 5 percent on a monthly basis and 2 percent on an annual basis. Accordingly, we see uncertainty ranges of about 2.5 percent in the major energy flows at the top of the atmosphere (in parentheses in the graphic).
But what does this mean for the that relatively small claimed ‘imbalance’ responsible for global warming? The imbalance is ten times smaller than the uncertainty in the measurement of the major flows (incoming and outgoing): 0.6 w/m2 in a flow of 240 w/m2 or about two-tenths of a percent.
What it means is that the IPCC claims to identify and and attribute (to CO2) a global energy imbalance of a magnitude that is so small that — if it exists — it is fully explained by our ignorance/uncertainty about the precise values of the energy flows we are trying to balance.
This is one (of several) reasons I find the IPCC stories about global warming unconvincing. Something is responsible for the mild, gradual warming of the earth since the end of the seventeenth century. I accept that it could be represented by an imbalance in the earth’s energy budged. But the identification and attribution (to CO2 by the IPCC) of the cause is impossible when the imbalance represents only a small fraction of our uncertainty in the measurement of the energy flows.
As far as I can see from this paper (and others like it) there is no way to distinguish the contribution of CO2 from other factors that, from ignorance, we have to call ‘secular natural variability’ in the climate.