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For more than half a century, from a remote monitoring station atop Hawaii’s dormant volcano Mauna Kea, the National Oceanic and Atmospheric Administration (NOAA) levels of atmospheric carbon dioxide.

In June 2024, researchers carrying out this monitoring work something staggering. Carbon dioxide concentration levels had reached over 426 parts per million, a far cry from the . 

These levels marked an alarming increase. Last year was also the hottest year on record. 

“We must recognize that these are clear signals of the damage carbon dioxide pollution is doing to the climate system and take rapid action to cut fossil fuel use as quickly as we can,” said then-NOAA Administrator Rick Spinrad. 

As humanity reckons with a climate shaped by a legacy of burning fossil fuels, we have clean energy technologies and even innovations that can pull carbon from the air. But implementing these solutions requires large-scale coordination and overcoming social challenges that stunt action. Research on Earth’s deep climate past shows just how much is at stake if we do nothing.

“We need to move away from fossil fuels, shut down our coal-fired power plants, use less oil and less natural gas,” said , a professor in the Department of Chemistry in the College of Letters and Science at 51�ԹϺ��� Davis. 

There’s really no future for fossil fuels, and that leaves us with nuclear and renewable energy, like wind and solar. — Frank Osterloh

The human imprint of climate change through carbon dioxide emissions

Humanity’s environmental imprint on the Earth is undeniable. Carbon dioxide emissions are a primary driver of climate change, which are by burning fossil fuels.

There’s a good reason why we have been burning fossil fuels for more than a century: they are an incredibly powerful source of energy.

“One awesome thing about petroleum fuels is that they pack a lot of energy into a small volume,” said , a professor in the Department of Economics. “It's a very energy dense fuel relative to other things that we might use.”

The transportation sector puts this energy density into perspective. A put the energy density of batteries at roughly 300 Watt-hours per kilogram, an increase of nearly one third over the past decade. Jet fuel has about 40 times that energy density. 

But this form of energy comes with a high cost. 

For , a Distinguished Professor in the , reaching a carbon dioxide concentration of 426 parts per million, a density the Earth hasn’t reached in , marked a tipping point.  

Four hundred and twenty-six, that’s not coming down any time soon,— We’re now at the point where the warming is sufficiently fast enough and intense enough that we’re releasing other natural sources of carbon. — Isabel P. �ѴDzԳٲ�ñ���

Recent environmental disasters such as wildfires, vast swings in temperatures and droughts can release this carbon and other greenhouse gases from bogs, permafrost, wetlands and other natural sources. These carbon stores occur naturally in the earth and oceans, and are a natural part of Earth’s biogeochemical recycling system. 

Add the legacy of humanity’s carbon dioxide emissions to the mix and you have the recipe for a compounding feedback loop. 

Even if fossil fuel emissions are drastically reduced, �ѴDzԳٲ�ñ��� said that the carbon dioxide humanity has emitted into the atmosphere will still require removal. 

However, a crucial limitation to adopting green technologies is the lack of infrastructure to support them. To make that energy production useful requires investments in green technologies and foundational research underlying them. 

“We have to try to electrify industrial production and switch to green hydrogen as a carbon-free fuel,” said Osterloh.

No one person can implement such changes. Rather, they require collective action from humanity. 

The collective action problem of climate change and carbon dioxide emissions

Climate change is at its core a collective action problem. Taking action on it comes with costs that not everyone who benefits will share. Economists call this the “free rider” problem.

“The costs are local and the benefits are largely global,” said Muehlegger.

At the same time, collective action problems are not unsolvable. 

When scientists discovered that the hole in the Earth’s protective ozone layer over the south pole was caused by the use of chlorofluorocarbons, or CFCs, the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer global agreement ended their use. In 2025, the hole in the ozone layer was the over the past 30 years.

One reason these agreements worked was because they involved a limited number of parties. Companies in only a handful of countries made products with CFC aerosols, so a global ban addressed the root cause.

“Some sectors are more concentrated with fewer stakeholders to deal with,” said Muehlegger. “With many stakeholders, as one party defects more parties are willing to defect, and then it's just very hard to keep consensus around these issues.”

Defections among multiple stakeholders was part of why this year’s . Global consensus broke down, not only about how to address climate change but also whether nations should take action at all. 

Solutions to reduce carbon dioxide for a sustainable future

In 2014, Toyota Motor Corporation unveiled the Mirai, a car that runs completely on hydrogen fuel cells. The initial sticker price was more than but today you might find the car on Craigslist for less than .

“It’s a fraction of its price because it’s difficult to find the hydrogen fuel for it,” Osterloh said. “There’s only one fueling station in Sacramento and they charge $35 per kilogram of hydrogen, enough for about 40 miles.” 

That makes driving a Mirai eight times more expensive than driving a gasoline-powered car.

“This shows that we somehow have dropped the ball on helping this technology and making cheap hydrogen available,” Osterloh said.   

Osterloh said there’s broad consensus around the world that green hydrogen is the fuel of the future. Green hydrogen is formed by water electrolysis, a process that splits water into hydrogen and oxygen with electricity from renewable resources, such as wind, solar or hydropower.

“It’s carbon-free and does not cause climate change,” Osterloh said. “China has taken the lead here as they now have over 50% of the world’s installed electrolyzer capacity and they also are the main producer of solar cells.”

But there are also non-photovoltaic ways to convert solar energy into green hydrogen fuel.

Osterloh and colleagues recently describing a new way to fabricate metal oxynitride materials, a class of semiconductors that shows promise for photoelectrochemical water splitting.  

“Photoelectrochemical water splitting is a next-generation technology that turns sunlight and water into hydrogen fuel directly,” Osterloh said. “It’s just one step. But you need very stable semiconductors for it and we are aiming to develop these materials.”   

Osterloh and his team recently filed a patent for their improved fabrication of oxynitride semiconductors. He views the solar technologies he’s developing and improving as pieces of the puzzle in mitigating climate change. 

“Solar water splitting with our semiconductors has a Technology Readiness Level (TRL) of 4-5,” said Osterloh, meaning they are between basic validation and prototype demonstration. 

On the other hand, solar cells, wind turbines and water electrolyzers to make hydrogen have a TRL of 9. They are ready to use. What’s missing in the U.S. right now is the political will for large scale implementation. 

Carbon sequestration to improve soil health and agriculture

In recent years, �ѴDzԳٲ�ñ��� has shifted part of her research to decarbonization efforts. 

“We have to get aggressive, not just about alternative energy, but we must get equally as aggressive about taking out this legacy carbon,” she said. 

With expertise in carbon cycle geochemistry, she’s partnered with soil science professor , a professor in the to explore the efficacy of using silicate rocks as a way to sequester carbon from the atmosphere. As silicate rock breaks down in the soil, it naturally absorbs carbon from the air in a process called chemical weathering.    

Not only does this practice sequester carbon, it improves the health of soils, leading to better water-holding capacity, more micronutrients and a neutral pH, all ideal for crop and forage growing conditions.  

�ѴDzԳٲ�ñ��� and O’Geene are currently conducting a multi-year, comprehensive study to measure just how much carbon is sequestered in agricultural soils infused with silicate rock. Recently , but it’s still being supported by a couple of 51�ԹϺ��� Davis donors. O’Geene said that funding for the study is committed for at least five years.

�ѴDzԳٲ�ñ��� said that industry is also stepping up. As an unpaid advisor for Microsoft, she’s working with the company to further develop criteria for large-scale deployments of enhanced rock weathering.  

“I don’t think it’s going to lose traction. It makes for healthier soil, higher crop yields and better water retention in the soils,” �ѴDzԳٲ�ñ��� said. 

As industry experiments with scaling up enhanced rock weathering solutions, the foundational research — like that performed by �ѴDzԳٲ�ñ��� and O’Geene — is happening in tandem.   

“It’s scaling faster than the science, and our research is helping to do better-quality scaling,” �ѴDzԳٲ�ñ��� said. “It’s been fantastic, but it’s also incredibly challenging and complex.” 

Soils are really complex, under the hood of the topsoil, it’s one hell of a zoo in there. — Isabel P. �ѴDzԳٲ�ñ���

After all, the real world is never as clean as the laboratory. The constituents of soil — the microorganisms and the minerals — are ever-shifting and vary by locale.  But dedicated study of these nuances is worth the investment. 

“Twenty-five percent of the Earth’s land could have this work for it and the method holds promise to potentially remove the equivalent of agricultural emissions of carbon dioxide to the atmosphere each year,” �ѴDzԳٲ�ñ��� said.

Government regulations to curb carbon dioxide emissions

One way for national governments to drive change in carbon emissions is through the tax system. A carbon tax, which imposes a cost on businesses based on their level of carbon emissions, has the advantage of reaching every industry nationwide. 

"There are going to be some sectors where carbon is reduced a lot and other sectors where carbon might not be reduced very much and that's exactly what you would want to happen,” said Muehlegger. “You want to capture the low-hanging fruit and not worry as much about parts of the economy that find it really hard to move away from carbon emissions.”

This has been a particular challenge for auto manufacturers with U.S. fuel efficiency standards that have changed with every president for the past decade. In 2012, the Obama administration increased , but in 2020 the first Trump administration . In 2022, the Biden administration enacted more aggressive fuel efficiency standards that the second Trump administration has to roll back as well. 

If companies invested in building cars with better fuel-economy in response to new standards, undoing those new standards might mean wasted investments.

“Although businesses prefer lower carbon taxes to higher carbon taxes, they also like regulatory certainty so they know exactly where they stand,” said Muehlegger.

The federal government isn’t always the only source of regulatory certainty. In the 1980s, California was the first state to pass tailpipe emissions standards that went beyond Environmental Protection Agency (EPA) standards. This was possible because the state received an EPA waiver to go beyond federal standards. 

Automakers found it cheaper to simply build cars to meet California standards rather than produce two completely separate lines, one for California, now the world’s fourth-largest economy, and one for the rest of country. 

“There are these moments where California's policy has basically pushed policy for the entire country forward,” said Muehlegger, “Not because California raises its standards and then federal standards rise to meet them, but because it’s easier for industry to meet just one set of standards.”

How ancient climate calamities set the stakes of climate change action today

Though the Earth’s average temperature has continued to rise, data show that carbon emissions across many of the world’s largest countries have leveled out, according to . Carbon emissions from the U.S. have actually been in decline for the past decade. However, China’s carbon emissions, for all its investments in green energy, have roughly tripled since the year 2000.   

Muehlegger and David Rapson, also a professor of economics at 51�ԹϺ��� Davis, recently conducted a study of what it would actually take for only the transportation sector to go fully carbon free at a global scale. Transportation accounts for 21% of global carbon emissions.

Even if the U.S. and all the world’s richest countries completely eliminated their carbon emissions from transportation, carbon emissions globally are still likely to rise.

“To some degree we should be proud that carbon emissions have not increased,” said Muehlegger. “But even if we reduce carbon emissions from transportation in the developed world, a substantial amount of future carbon emissions and climate change is going to happen in the developing world as these economies become larger and their citizens become wealthier.”

The Earth’s deep past gives context to this climate present and, if we do nothing, our potential future.  

“The Earth was warmer before, there was more carbon dioxide before, and those dramatic increases are commonly associated with mass extinction events,” said , an assistant professor of earth and planetary sciences. “Those deep-past changes are nowhere near our current carbon dioxide increases.” 

The reason why our super-charged carbon emissions puts at so much risk even compared to ancient climates is that Earth’s systems are like a series of dominoes set in a circular pattern. If one falls, the others can topple, but every single domino is a prime mover. When one falls, regardless of which one it is, it can affect the entire system. 

Take the ocean current known as the Atlantic Meridional Overturning Circulation (AMOC), for instance. from Sun and colleagues indicates that even a modest slowdown of this current, which transports heat and energy from the tropics to the North Atlantic, could dry out rainforests, threaten vulnerable ecosystems and upend livelihoods across the tropics. A from the Intergovernmental Panel on Climate Change suggests that future warming will very likely compromise the AMOC.

Because climate is so dynamic and propagates globally, Sun said ripple effects from an AMOC slowdown would be felt worldwide.

Based on 17,000 years of paleoclimate records, Sun and his colleagues predicted that an AMOC slowdown would lead to temperature drops in the tropical Atlantic and Caribbean, leading to significant reductions in precipitation in Central America, the Amazon and West Africa. 

“This could affect billions of people living in those regions,” he said. 

Even further back in time, an increase in atmospheric carbon dioxide 300 million years ago led to significant decreases in ocean oxygen by about 4% to 12% for periods lasting hundreds of thousands of years, according to from �ѴDzԳٲ�ñ��� and colleagues. 

While the environmental context isn’t one-for-one, a modern decrease in ocean oxygen levels of a similar amount would devastate marine life and fisheries. With more than for a significant percentage of their protein, what happens to those people if these fisheries collapse?

Already, European Commission data indicates that the heaviest burden of not addressing carbon emissions would fall to countries outside of the that belong to the Organisation for Economic Co-operation and Development (OECD). 

This year’s COP30 meeting in Brazil saw people representing Indigenous communities . The roughly 100 peaceful protesters called focus to the Amazon rainforest, pushing back against development projects that would lead to deforestation in the region. 

These are the people immediately facing the repercussions of our climate crisis. But eventually we will all experience it in some shape or form. Whether it’s hotter and drier summers or increased prevalence of wildfires or shrinking populations of marine life on the coast, the effects will be a threat to everyone. 

Unless we all take action. A worldwide problem requires truly global solutions with multiple people, communities and governments working together. 

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