On the sidelines of the 2015 Paris climate summit, then-UK prime minister David Cameron and 19 other world leaders made a promise to double green energy research and development by 2020.

The United Kingdom is on course to break that promise. As a percentage of GDP, spending on low-carbon energy R&D has stayed sluggishly around 0.02 per cent since 2015, according to data from the International Energy Agency (IEA).

The United Kingdom is not alone. IEA data shows rich OECD countries are spending just 0.03 per cent of GDP on low-carbon energy R&D – a percentage that has not changed since the vow was made.

Climate policy has been littered with broken promises ever since the 1992 Earth Summit in Rio de Janeiro. Indeed, the core promises written into the Paris Agreement are not being met. A recent study revealed that only 16 countries — the likes of Samoa and Algeria — are living up to their vows to reduce carbon emission growth, and they’re only doing so because they promised to do very, very little.

But the R&D promise is different. It matters that the UK and rich countries are ignoring this commitment, because investment in green energy is the single policy that could make the biggest difference against climate change.

Carbon-cutting promises fail for the very reason R&D promises need to be met. Green energy is far from ready to take over from fossil fuels, so vowing to cut emissions usually just means choking growth, which rarely succeeds.

The reality is that today, solar and wind energy together only deliver about 1 per cent of global energy. The International Energy Agency estimates that even by 2040, these will cover just over 4 per cent of global energy.

Despite governments encouraging the roll-out of green energy technology, renewables still have two big problems.

First, they take up an amazing amount of space which often replaces nature. To produce the energy equivalent to a one-hectare gas-fired power plant you would need 73 hectares of solar panels, 239 hectares of on-shore wind turbines, or an unbelievable 6,000 hectares of biomass.

Second – and more importantly – solar and wind power are intermittent or unreliable. Solar energy isn’t produced when it is overcast or at night-time. Wind energy isn’t produced when there is little or no wind.

We often hear that wind and solar energy are cheaper than fossil fuels, but at best that is only true when the wind is blowing or the sun is shining. On a windless dark night, the cost of wind and solar power is literally infinite. That is also why it is deeply misleading to compare the energy cost of wind or solar to fossil fuels only when it is windy and sunny.

Since all solar energy is captured around the same time, the value quickly drops dramatically — one study shows that in California, when 30 per cent of the market share is solar energy, it will lose two-thirds of its value.

What’s more, because modern society requires 24 hours of power, even when solar or wind is introduced, it’s still necessary to pay for back-up service from fossil fuels (for when there’s no wind or sun). Only, these are now more expensive because fossil fuels have fewer hours to pay back the capital.

Battery technology is far from ready to help solar and wind energy last longer: in the US, total battery storage could power the nation for just 14 seconds.

The reality is that alternatives to fossil fuels just don’t cut it. And this reality goes a long way to explaining why carbon cutting promises have failed – and have failed to rein in temperature rises.

For almost anybody on the planet, getting access to more energy means more income, longer lives, less disease, and more education.

Typically, despite 30 years of climate policies, the cheapest way to achieve this is still through coal. The International Energy Agency‘s newest report finds that when adjusting for the unreliability of solar and wind, existing coal will be cheaper than new solar and wind in all the major regions until at least 2040.

This simple fact is the reason we do not yet have a solution to global warming: green energy mostly can’t yet compete globally with fossil fuels.

This is why, when 27 of the world’s top climate economists and 3 Nobel Laureates looked at the whole gamut of climate solutions for the Copenhagen Consensus, they found that green innovation is the best investment.

Investing dramatically more into green energy R&D means we can start looking for lots of solutions. It could mean better solar and wind, combined with batteries. We certainly should research those areas further (rather than erecting masses of inefficient solar panels and wind turbines today). But we also need to focus on exploring fusion, fission, water splitting and many other ideas.

David Cameron’s promise in 2015 should be more than just empty words. It committed the country to a pathway toward finding cheaper and better alternatives to fossil fuels. Three years have passed with little to show for it. It is time to recommit to that goal.