The IMF estimate they're about
$1.9 trillion/year. You'll have to dig around for their methodology.
This discussion reminds me of an interesting blog post from January:
High-stakes climate poker
The fossil fuel industry is betting that we'll keep pumping it money instead of paying less to switch to renewable energy
The bottom line is: we give the oil and gas industry $1.9 trillion every year in subsidies, when it would only actually cost $850 billion per year to switch to a renewable energy economy. And that doesn't even take into account the health costs of fossil fuels due to air pollution ($300-900 billion per year just in the USA!).
The status quo is totally sub-optimal
But that's not the UK is it... Much of that so called subsidy is also not a subsidy, (being reductions in tax that only the oil and gas companies pay - if that counts then governments pay a 100% subsidy on fresh fruit...). The vast majority of the subsidy is not going to the oil and gas industry directly, it's reducing the price of fuel for citizens and farmers in developing nations, a prime example being Venezuela. If they don't subsidise it people cannot afford to work or pay for the inevitable significant increase in food and transport costs (and you end up with rioting and protests).
There in lies the problem. We can reduce the "subsidy" in the UK by getting rid of the 5% VAT rate on domestic gas and electricity, replacing it with the standard 20%. That'll go down well. All you are actually doing is increasing the green tax for consumers. Obviously you also have to remove the tax paid by oil and gas producers from that sum as well, which in the UK varies between about 30% and 90% of the profits from the sale of hydrocarbons (on top of standard corporation tax).
Internationally it may work out a bit better due to countries actually putting their hands in their pockets and physically subsidising fuel. Unfortunately its not that simple either. Aside from the inevitable riots and starvation between the cutting of the subsidies and the renewable fuels taking over you have the issue that most of the subsidies are paid for by the tax collected from the oil companies in those countrkes in the first place.
Basically the idea of just taking $1.5t and spending it on renewables rather than "giving" it to oil and gas companies doesn't work because the countries don't have the money in the first place.
I agree however that we need to be spending more money on carbon reduction, whether that be renewables, sequestration or just reducing our consumption in the first place. Unfortunately the oil and gas subsidy argument doesn't really hold water with me.
Edit: what we really need to sort out is the carbon tax. We had the chance to make it so much better by setting up a system that was flexible and persuaded companies to reduce their carbon emissions or force them to increase the price of their goods. Unfortunately the European Parliament blew it and now we still have a system that prices carbon way too low, to the point companies aren't bothered with the extra cost. I guess the positive is we still have a carbon tax system, unlike the Australians who just lost theirs.