Greenhouse gases, sulfates, and the long-term warming trend

Earth has warmed. How do we know, and what caused it? Keep those questions separate. The opening supports the warming itself by comparing recent temperatures with a much longer record. Geological patterns, ice, tree rings, and coral help researchers reconstruct earlier temperatures. Against that background, the half-degree Celsius rise over the previous century stands out: the increase is unprecedented in the preceding thousand years.

Now we need an explanation. Proponents of the greenhouse effect point to gases that keep heat from radiating back into space. More of those gases could make the atmosphere warmer. That makes sense as a proposed cause, but does it account for what scientists actually measure?

The early models had a problem. They calculated more warming than had occurred. If the theory is good, why don't the calculations line up with reality? That discrepancy gave opponents a reason to question the explanation. The author needs to address it, not just repeat that greenhouse gases trap heat.

The missing factor was airborne sulfates. They reflect solar energy back into space, counteracting some of the warming. We have two effects working in different directions: greenhouse gases help retain heat, while sulfates reflect energy away. If a calculation includes the heating influence but leaves out the cooling influence, it can predict too much warming even though the heating influence is real.

Scientists use sulfate concentrations in layers of glacial ice to estimate past sulfate levels. Once they include that information, the calculated temperatures agree with the observed temperatures. That is the repair. The models needed a factor they had overlooked; the improved fit gives the author a reason to defend the greenhouse explanation. It does not mean sulfates are themselves the greenhouse gases.

The next paragraph raises a different possibility: what if variations in the sun explain the increase? Give this alternative its due. The models show a strong decade-by-decade connection between solar activity and temperature fluctuations. The author does not deny that the sun affects temperatures. The question is whether those variations explain the entire recent rise.

They don't, according to the account here. Temperatures have moved up and down around a relatively stable long-term average. Those earlier departures returned toward that average. The current rise goes beyond even the most extreme fluctuations the solar models can explain. Fluctuating around the old average is different from the average itself shifting upward.

That distinction takes us to the conclusion: changes in the atmosphere appear to have raised the equilibrium temperature, and greenhouse gases offer the best explanation. A cause that explains some short-term variation need not explain the whole long-term trend.

So both challenges end up supporting the same direction. The sulfate evidence answers the problem with the greenhouse calculations. The solar discussion shows why an alternative explanation is insufficient. We should leave with a defense of greenhouse gases as a major cause of the warming, not with a forecast of future temperatures or a claim that every other influence has disappeared.