They are absolutely not a new natural phenomenon, however fire in the environment became far more common when modern humans (and even pre-modern humans) came into the picture, and will continue to become more common as anthropogenic (human created) climate change becomes more pronounced. As a British archaeologist, I can give the British perspective however I should imagine that the phenomenon is more widespread than just the British Isles.

In Britain (as elsewhere) you have sediments which have been naturally deposited over a very long time. These sediments form a record which can go back tens of thousands of years ago (or more!), sometimes over multiple phases of natural environmental change such as glaciations, interglacials, small warm episodes, short-lived cold phases, etc. Within these sediments you can find microscopic remains such as pollen, single-celled organisms called forams, or the remains of small creatures called ostracods. These are all reflections of the local environmental conditions at the time the sediment was deposited. You can also get along with these microscopic fragments of charcoal (aka microcharcoal).

This microcharcoal is taken as a proxy for fires which are occurring in the nearby landscape and you can find this microcharcoal sporadically at low levels going back through millennia of sedimentary deposition as a result of natural fires from lightening strikes etc. There is, broadly, the understanding that increases in microcharcoal can be taken for evidence of increased aridity and temperature (linked to climate) as in theory this would lead to increases in wildfire frequency. There are efforts to tie in climate variation as defined by chemical isotopes within ice core records in Greenland to increased microcharcoal concentrations in terrestrial sediment records as a way to demonstrate the above link between microcharcoal and climate change. Plus, you often find microcharcoal along with the other microscopic remains I mentioned above that give indications of local environmental conditions which in turn are influenced by climate change.

Given that we broadly understand that increases in microcharcoal (as a proxy for wildfire frequency) generally occur during warmer climate episodes, it is sensible to conclude that as anthropogenic climate change occurs we will see increases in wildfire.

What is also very clear is that within the sediment record it is very common that as soon as humans move into an environment following the Neolithic revolution (and the invention of agriculture and, very arguably 'society' and 'civilisation') microcharcoal concentrations increase dramatically within a landscape. This is due to a combination of humans burning wood but also due to humans causing fires within a landscape- both intentionally and unintentionally. At no point during the last 2.4 million years has there been an increase in wildfire frequency to the extent that we have seen since the Neolithic revolution. Given that there has already been a large increase in wildfire frequency even before anthropogenic climate change, future wildfires will become even more unprecedented in their scale and frequency.