Origin of Serious Wildfires At a very rough estimate, what proportion of wildfires are caused by natural events like lightning? I am absolutely sure that in Europe, where there are often wildfires, the sun is never hot enough to set fire to vegetation directly, though it obviously makes things tinder dry and easily combustible. The broken bottle theory has been disproved. Experiments involving broken glass among highly combustible straw or hay were unsuccessful in causing combustion, though manual focussing of the suns rays with the lens-shaped bottom of a bottle were on rare occasions and with much difficulty successful. Although lightning undoubtedly causes the occasional wildfire, the majority have no connection with lightning. It should also be remembered that lightning is usually accompanied by torrential rain. These fires often start in several places at once, which is stretching the long arm of coincidence a bit too far. 

Apart from the very rare instances of volcanos causing wildfires, what other natural causes could there be?
Comparison of wildfire frequency in tinder-dry uninhabited areas, well away from civilisation, versus frequency in or near similar but populated areas should give a good idea of the scale of the problem.
There was an interesting paper on this for the United States a couple of years ago:

Balch et al (2017) Human-started wildfires expand the fire niche across
the United States, PNAS, doi: 10.1073/pnas.1617394114

They estimate that in the coterminous U.S. human-started events account "for 84% of all wildfires and 44% of total area burned".  There is a marked spatial variation, with lightning-started wildfires occurring mainly in the more sparsely populated western forests.  If you have access, the paper's worth a read, it has lots of interesting figures picking apart the details.



It's worth noting that there are a number of reasons for human-started wildfires, which are could be classed as accidental, deliberate or careless:


  Human-started wildfires were caused by a variety of sources, including the US Forest Service-designated categories of equipment use, smoking, campfire, railroad, arson, debris burning, children, fireworks, power line, structure, and miscellaneous fires.


They also note that their data contains this, perhaps not unsurprising, feature:


  There was also a notable mark of American culture on the distribution of wildfires, with the peak day of wildfires occurring on July 4th, concurrent with Independence Day fireworks displays (Fig. 2). Indeed, Americans start over twice as many wildfires on July 4th as any other summer day.