Scientists don’t understand the climate well enough to quantify precisely the relative contribution of the various human and natural factors in the current warming trend

See Is Anything Wrong With Natural, Non-Man-Made Climate Change? by Mario Loyola. Mr. Loyola—author of "Twilight of the Climate Change Movement"—is senior fellow at the Wisconsin Institute for Law & Liberty. Excerpts:

"Many climate alarmists, however, are like the fellow who unwittingly admitted that he’s not actually alarmed about climate change in and of itself, just climate change caused by human activity. The most radical of these environmentalists flatly deny that temperatures and sea levels could be rising partly for natural reasons. In other words, they deny natural climate change. Call them “climate deniers” for short, since they are denying that the climate is doing now what it has always done, namely change for natural reasons.

Ironically enough, it turns out that these climate deniers are also science deniers. The UN’s Intergovernmental Panel on Climate Change (IPCC) summarizes all of the climate science that climate alarmists use to justify their anti-carbon policies. It is the most authoritative source for environmentalists’ claims about the scientific consensus on climate change. On the link between human activity and climate change, the IPCC has this to say: “It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.” (Emphasis added.)

The same IPCC report says that current warming is “unequivocal” i.e.; there is virtually no doubt that the planet is warming. But the IPCC is not nearly so unequivocal about the causes. It cites human activity as the major cause of warming, but not necessarily the only cause. Scientists aren’t sure what the climate trend would be in the absence of human activity; it’s possible that carbon emissions have an even bigger warming impact than they fear, and the impact is being mostly absorbed by an underlying cooling trend; they just don’t know. The IPCC’s carefully qualified attribution statement recognizes that scientists don’t understand the climate well enough to quantify precisely the relative contribution of the various human and natural factors in the current warming trend.

The bottom line is that scientists are much more confident that the planet is warming than they are confident that they understand why the planet is warming. This only stands to reason. It is obviously easier to measure temperature change than to draw “unequivocal” conclusions about causation from the incredibly rich, complex, and often impenetrable picture that the climate data present. Those who think that the scientific debate is over are the real science deniers.

Uncertainty is not necessarily fatal to precautionary policies such as the widespread calls for reducing carbon emissions. Policies designed to guard against risks have to take uncertainty into account. But uncertainty is not an excuse for throwing rational cost-benefit analysis out the window. Through policies like the Paris Agreement on climate change, alarmists are proposing hugely expensive reductions in carbon emissions that would hit the world’s poorest and most vulnerable populations hardest. But the only benefit they propose is a reduction in warming that today’s scientists would not be able to measure, much less conclusively attribute to the policy. Warming could stop completely without any reduction in carbon emissions, and it could continue despite the elimination of all carbon emissions. Scientists don’t know what the future holds because they don’t understand natural climate variability well enough to say what the underlying climate trend would be today in the absence of human impact.

It’s very telling that climate alarmists never mention natural climate change. And yet the danger of natural climate change is all too real. Most people don’t realize that the last 9,000 years have been uncharacteristically stable compared to the violent climate changes in the 9,000 years before that. 18,000 years ago, the state of Wisconsin was under nearly two miles of ice. Average temperatures were 40 degrees Farenheit lower than they are today, when they suddenly began to soar. The glaciers that covered most of the northern hemisphere started melting away, and never stopped melting. Ocean levels rose 300 feet between 15,000 and 8,000 years ago; that’s less time than between Sumerian civilization and the present day. It is very likely that we are towards the end of a short warm period between major glaciations of the Pleistocene Ice Age, which has lasted 2.6 million years. Carbon dioxide levels are the highest they’ve been in 800,000 years, as we’re often told, but the baseline is that of a ice age that has brought carbon dioxide levels to their lowest point in 245 million years."