Royal Dutch Shell has long been one of the dominant players in the global oil market, but it apparently sees a future in natural gas. On Wednesday, Shell announced it will buy BG for $70 billion, a move that will add considerably to the oil giant's capacity and profits in two areas: deepwater drilling and liquefied natural gas (LNG).

As the market for natural gas has taken off, companies have increasingly turned to cooling it into liquid form, so it can then be transported while taking up less space. Shell has pumped tens of billions of dollars into the necessary plants, terminals, and other infrastructure, according to The New York Times.

But if you go through the deal, Shell's logic is short-term and reactive. If the company wants to think truly big, it's going to need to supplement its LNG investments with what has long been assumed to be the bane of fossil fuel companies: renewable energy.

Part of the deal's attraction is the precipitous drop in oil prices we've seen since mid-2014. Since both Shell and BG are also into oil in a big way, that's allowed Shell to snap up BG's assets at relatively cheap prices, despite paying 50 percent more than BG's closing share price on Tuesday. The consolidation will ostensibly allow Shell to combat those falling prices by eliminating overlapping costs, and to immediately expand its oil and gas reserves by 20 percent while upping annual production by 20 percent. The LNG market is also expected to keep expanding, as China, in particular, moves off coal to reduce air pollution.

Now, the price of oil is not going to stay this low for long. The global middle class is exploding — see, again: China — which is driving demand through the roof. At the same, there's only so much oil on the planet. We've been extracting the reserves that are easy to reach, and now we're getting into the truly difficult stuff. Look no further than Shell's own tragicomic attempts to drill in the unforgiving environs of the Arctic Circle.

The price of natural gas is under essentially the same pressure. Studies of the various U.S. fields where horizontal drilling and hydraulic fracturing have led to the boom in domestic oil and gas production suggest the wells may economically tap out within a decade. As the geologic pressure in the shale formations drop, companies have to keep drilling more and more wells to maintain the same level of production, and eventually the costs overwhelm the benefits. Another study found much the same problem at the global level.

Then there's climate change. President Obama's official pledge is that America will cut its carbon emissions by 26 to 28 percent by 2025. China has agreed to get 20 percent of its energy from green sources by 2030, and to peak its carbon emissions that same year. And the European Union is currently passing legislation to slice its emissions down by 40 percent by 2030.

The simple fact is that if the world is going to avoid a level of global warming that most scientists agree will be catastrophic, then fossil fuel companies like Shell are going to have to leave an enormous portion of their proven reserves in the ground.

In short, Shell faces a closing window of opportunity, in terms of the physical and economic limits of fossil fuels, and how long the global market will keep paying for them. So Shell needs an out, and that out is green energy — solar and wind specifically.

There is a natural symbiosis that can occur between natural gas and renewables. Despite having rather high energy prices, Germany, for example, has demonstrated that sufficient coordination of renewables like solar and wind can yield a remarkable level of grid reliability. But there will still be gaps to fill in, due to peak demand times for energy or just the natural unpredictability built into relying on the sun and the wind for energy. Natural gas is well-suited to that role, because power plants that rely on it can be fired up or powered down relatively quickly in response to need.

Meanwhile, the falling costs of battery technology plus the falling costs of solar are likely to make home solar systems with storage a better deal than traditional grid power in about a decade. Analysis suggests the price of solar power will achieve near-global competitiveness with natural gas prices in about the same time, and the pace at which solar capacity is being adopted globally is blowing away projections that are just a year old.

In the car market, meanwhile, hybrids and battery-powered vehicles that maintain a gasoline engine as a backup are likely to keep growing as the best way to straddle the divide between reliance on oil and reliance on the grid.

(One major caveat is that methane, which makes up most natural gas, is itself a powerful greenhouse gas. Analysis of the natural gas industry shows that leaks of methane may very well be so severe that they completely offset the climate value of burning natural gas rather than oil or coal. So policymakers will need to crack down with stringent standards and regulation.)

Shell can roll with these forces rather than against them. It could position itself as the supplier of both green power and natural gas and oil as the world enters its transition phase to a (hopefully) fully green energy economy over the course of this century. Major solar manufacturers like Trina Solar and Canadian Solar, as well as American companies like First Solar and SunPower, are on the rise, but can still be bought for a fraction of the price Shell just dished out for BG.

And when it's over, we could look back on Shell as an instrumental player in the fight to stave off climate change.