US Constitution. Article 1, Section 10.
> Brown later held a closed-door meeting with the Chinese president, Xi Jinping, during which the two pledged to expand trade between California and China with an emphasis on so-called green technologies that could help address climate change, Brown said.
From the Constitution, article 1 section 10:
> No State shall, without the Consent of Congress, lay any duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
There's the out. It may (and will) be argued that climate change is an imminent Danger that will not admit of delay. The associativity of the clauses is just ambiguous enough that the last one can be paired with any of the others, not just engaging in war. And, I am confident there are people in California willing to take that argument to the Supreme Court.
Not well-read on the case law here, but are you sure signing a non-binding pledge qualifies under this section? I would assume it underwent basic vetting by constitutional law experts.
They have determined that it does not cover every agreement or compact:
---------Virginia v. Tennessee 148 U.S. 503 (1893)---------
There are many matters upon which different states may agree that can in no respect concern the United States. If, for instance, Virginia should come into possession and ownership of a small parcel of land in New York, which the latter state might desire to acquire as a site for a public building, it would hardly be deemed essential for the latter state to obtain the consent of Congress before it could make a valid agreement with Virginia for the purchase of the land. If Massachusetts, in forwarding its exhibits to the World's Fair at Chicago, should desire to transport them a part of the distance over the Erie Canal, it would hardly be deemed essential for that state to obtain the consent of Congress before it could contract with New York for the transportation of the exhibits through that state in that way. If the bordering line of two states should cross some malarious and disease-producing district, there could be no possible reason, on any conceivable public grounds, to obtain the consent of Congress for the bordering states to agree to unite in draining the district, and thus removing the cause of disease. So, in case of threatened invasion of cholera, plague, or other causes of sickness and death, it would be the height of absurdity to hold that the threatened states could not unite in providing means to prevent and repel the invasion of the pestilence without obtaining the consent of Congress, which might not be at the time in session. If, then, the terms "compact" or "agreement" in the Constitution do not apply to every possible compact or agreement between one state and another, for the validity of which the consent of Congress must be obtained, to what compacts or agreements does the Constitution apply?
We can only reply by looking at the object of the constitutional provision and construing the terms "agreement" and "compact" by reference to it. It is a familiar rule in the construction of terms to apply to them the meaning naturally attaching to them from their context. Noscitur a sociis is a rule of construction applicable to all written instruments. Where any particular word is obscure or of doubtful meaning, taken by itself, its obscurity or doubt may be removed by reference to associated words, and the meaning of a term may be enlarged or restrained by reference to the object of the whole clause in which it is used.
Looking at the clause in which the terms "compact" or "agreement" appear, it is evident that the prohibition is directed to the formation of any combination tending to the increase of political power in the states, which may encroach upon or interfere with the just supremacy of the United States.
But what if California actually then goes on to enforce the agreement by forcing, say, utilities to do things which they object to? I suspect the courts would at least demand that California pass a statute in order to accomplish that, and then the fact that an agreement with a foreign power motivated the statute might be of little interest. Who knows! Fun times.