When produced and used properly, hydrogen can potentially play many roles in the transition toward clean energy and industrial systems. Hydrogen can directly replace natural gas and coal in industrial and manufacturing processes where high temperatures are needed, replace natural gas and heating oil to keep buildings warm, and replace gasoline and diesel fuel to power cars and trucks.

When combined with CO₂ captured directly from the air, hydrogen can be used to manufacture carbon-neutral synthetic fuels, which could replace gasoline, heating oil, diesel and jet fuel, as well as forming the feedstock for carbon-neutral chemicals and plastics.

And hydrogen can be used as a storage medium for electricity, outperforming batteries when it comes to storing large volumes of electricity over monthly or seasonal time scales. But hydrogen also poses several challenges.

The first is storing it. Energy is required to pressurize or chill the hydrogen. The small molecules tend to diffuse through many materials. There are promising new options, making use of nanomaterials and conversion of hydrogen into other chemicals for its storage, but these technologies are not yet mature. For now, storage costs are high.

The second challenge is producing it. Currently, nearly all hydrogen is produced from natural gas, releasing large volumes of CO₂ into the atmosphere. This is known as gray hydrogen and must be phased out because of the CO₂ emissions it causes.

Blue hydrogen is also produced from natural gas, but the CO₂ is captured and permanently stored underground rather than being vented. Unfortunately, this results in large quantities of leaked natural gas, which is primarily methane—a powerful greenhouse gas.

The cleanest solution, green hydrogen, uses electricity from carbon neutral energy sources to split water into hydrogen and oxygen. But this is highly inefficient. By the time the hydrogen is ultimately consumed for heat or electricity, more than half of the energy content of the original electricity is lost (see also this blog post).

New concerns over hydrogen

Most recently, a third challenge has come to light: hydrogen leakage. Until recently, leakage had simply been viewed as an economic loss. But there is far more at stake. Leaked hydrogen reacts with scarce OH radicals in the atmosphere. That leaves fewer OH radicals to react with methane. Leaked hydrogen thus extends methane's atmospheric lifetime, worsening its effects on the climate.

Researchers examined these factors together to evaluate the climate benefits of various hydrogen usage scenarios. Scenarios involving only green hydrogen deliver strong climate benefits relative to the fossil fuels that are replaced, assuming hydrogen leakage rates to be low. But as soon as blue hydrogen is introduced to the mix, and hydrogen leakage rates are assumed to be higher, the benefits decline, and in some cases disappear altogether. For example, a scenario with 30% blue hydrogen and leakage rates exceeding 3% would lead to more warming over a 20-year period than the fossil fuels hydrogen replaces.

How much hydrogen would actually leak?

The answer is that we don't know, because very little research has been done. There is reason to believe that hydrogen would leak more than natural gas, and some estimates of natural gas leakage put it over 3%. The most comprehensive study to date estimates the likely leakage rate for hydrogen to be 2.9%–5.6% but acknowledges that it might be higher.