Climate change is extremely convenient for the left to promote leftist policies

In so far as this is true, it is also true that climate change denial[1] is extremely convenient for the right. (And vice versa.)

[1] I intend this here to mean simply "denying" rather than "denying in the face of what ought to be overwhelming evidence"; there doesn't seem to be a neutral way of putting it.

This is a general phenomenon: if there are rival positions X and Y on a factual matter, which if true would support rival positions P and Q on a matter of policy, then you may suspect partisans of P of bias when they assert X, but you may equally suspect partisans of Q of bias when they assert Y. So if you are not yourself very partisan, what difference should this make to your opinions about X versus Y? It should make you treat someone's opinion about X/Y as less informative in so far as they have a partisan position on P/Q that would explain it. (But bear in mind that the causation may go X/Y -> P/Q rather than the other way around, so the appropriate discounting is less than you might naively think.)

So, in this case, you might reasonably be very suspicious about American politicians' statements about climate change, because in the US the issue is very politicized. So, where else should you look?

• Outside the US. It's really only in the US that such a tight relationship between politics and opinion on climate change is present. Outside the US, even right-wing politicians (whom you might, in terms of the model here, expect to have a strong bias against belief in anthropogenic climate change) generally agree that the climate is warming because of human industrial activity, and they generally favour taking measures to reduce the warming. (But "conservative parties do not challenge coal or petroleum in countries with large reserves of these resources".)
• The opinions of actual climate scientists. Being a scientist is of course no guarantee of being unbiased, and there are ways for scientific institutions to be biased even when individual scientists are not.[2] But reducing the influence of bias is a large part of what scientific methods and institutions are for, and probably most climate scientists got into the field before it was as political as it is now. (And many of them are outside the US; see above.) Well, it appears that climate scientists are pretty close to unanimous about the reality of anthropogenic climate change.
• People whose opinions on politics and climate change don't "match". E.g., take a look at the results here. It appears that about 24% of conservative Republicans accept anthropogenic climate change, and about 19% of liberal Democrats don't. Maaaaybe that's evidence that anthropogenic climate change is real. (But you need to be careful about this sort of thing: it could be that the political bias is stronger one way than the other. For what it's worth, my guess is that on this issue the conservative->no-ACC link is stronger than the liberal->ACC link, which would mean that that discrepancy is evidence for ACC, but I don't have any very compelling arguments to support that guess.)

[2] E.g., a wealthy entity -- a government, an industry consortium, etc. -- might provide funding for climate research in ways that happen to favour employment of scientists whose views match the wealthy entity's. Those scientists may individually be perfectly unbiased, following the evidence wherever it leads, but the process that selects them may be biased.