Fossil fuels are organic material – they're made up of the remains of living things that, over hundreds of thousands of years, have broken down into oil and coal. When that fuel is burned, the carbon becomes carbon dioxide, which enters the atmosphere. 

But carbon dioxide is a greenhouse gas. This means it prevents heat from leaving the atmosphere. This is, to a point, a good thing. We need a little carbon dioxide in the atmosphere, because otherwise the Earth would be too cold to support life. But all of the extra carbon dioxide that we are adding traps too much heat, ultimately raising Earth's temperature. You can think of this like you would adding blankets to your bed: one blanket might be comfortable on a winter night, maybe two, but as you keep adding blankets, you reach a point where you start to feel uncomfortably warm. 

This is essentially what we are doing to our planet. And the more carbon dioxide we blanket Earth with, the more our finely-tuned climate system will change, with bad effects for human lives as well as the plants, animals, and ecosystems that we rely on. 

Changes in the atmosphere change Earth's reflectivity

One serious effect of trapping extra heat in Earth's atmosphere is that it will further change our planet's ability to reflect solar energy, by changing Earth's color. This ability is called albedo, and as more and more ice melts, Earth's albedo decreases, making it less reflective. 

As global temperature goes up, ice starts to melt at the poles. This means that regions of the world that were once covered in ice are now covered in dirt and soil (when land ice melts) or are exposed patches of ocean (instead of sea ice). The Arctic Ocean, for example, could become ice-free during the summer in as little as 15 years, exposing the ocean surface during the warmest part of the year. As anyone who's ever ridden in a black car on a hot day knows, darker colors absorb more heat, while lighter colors reflect heat. Ice is white, so very reflective, but both seawater and soil are dark, absorbing more heat. 

This is a positive feedback effect. As climate warms, more ice melts, and as more ice melts, climate warms. A positive feedback, left unchecked, will continue to intensify. 

Melting ice sheets raise global sea level

Melting ice does more than just change the color and reflectivity of the Earth. Sea ice melt in the Arctic changes the habitat of polar bears, Arctic foxes, and other animals, who use the sea ice to feed and find mates. And the melting of land-based ice sheets in Greenland and Antarctica adds more freshwater to the ocean, raising sea levels. 

You may have heard that if the Greenland and Antarctic ice sheets all melted, sea level would rise by around 70 meters, or 230 feet. This is a problem because about a third of the world's population lives on a coast and is in danger of having their homes and livelihoods swept away. For example, if greenhouse gas emissions don't change, Miami could lose between 30 and 70 percent of its total land area by 2100. 

But that isn't the only reason that coastal communities should worry about sea level rise. Some coastal areas might cope with sea level rise, only to be inundated with unexpectedly high storm surges during severe weather events. And that's an issue because storms like hurricanes and typhoons get their energy from the warm seawater they pass through, and in a warming world, hurricanes and typhoons are expected to increase in both frequency and intensity. 

This is already happening. For example, this year saw one of the strongest typhoons ever, Typhoon Rolly, hit the Philippines, costing $368 million in damages. Two weeks later, the country was again hit by Typhoon Ulysses, causing a further $278 million in damages. At least 98 people died and 400 were injured in these two storms.

Sea ice melt changes ocean circulation

Sea level rise isn't the only thing that is affected by climate change, though. Climate change also affects ocean circulation, the movement of water around the ocean. 

Normally in the Atlantic Ocean, warm salty water is brought north by the Gulf Stream, cooling as it moves, and begins to sink just off the coast of Greenland. This phenomenon, known as Atlantic Meridional Overturning Circulation (AMOC), is responsible for replenishing the North Atlantic's supply of deep water. This constant northward flow warms the water in the North Atlantic. It is also responsible for Western Europe's relatively mild climate. Edmonton, Alberta, for example, is at the same latitude as Hamburg, Germany, but experiences winter temperatures that are on average 10 degrees Celsius colder. 

But as ice caps melts, more and more freshwater will flow into the ocean south of Greenland. This could change the water's density. As seawater becomes less salty, its density decreases compared to normal seawater at the same temperature. Some researchers think this influx of freshwater would dilute North Atlantic seawater enough to disrupt AMOC.

If this portion of the ocean conveyor belt were to weaken or shutdown, warm water would no longer continue to flow into the North Atlantic. If this happens, sea surface temperatures in the area will drop, and Western Europe could become colder by about 1-5 degrees C. This means that Europeans will no longer be able to grow the same crops, and animals that live in Europe will have to adapt to cooling temperatures, migrate, or go extinct.

It might not seem intuitive that as global temperature warms, some areas of the Earth could grow cold. However, that's why it's important to look at the bigger picture. Overall, global temperature goes up, but regional and local effects depend on the factors that drive climate in these places. This is one reason why scientists now prefer the term "climate change," as opposed to "global warming" (which was popular a few decades ago), because a change in global average temperature doesn't just mean that everywhere in the world will warm. 

Carbon dioxide acidifies the ocean

Sudden influxes in freshwater aren't the only things affecting the chemistry of the ocean. The sudden increase of carbon dioxide in the atmosphere is having serious effects on the chemistry of seawater. Because the ocean and the atmosphere are in constant contact, about a third of the excess carbon dioxide that humans have emitted into the atmosphere has been absorbed by the ocean. This is a big deal for global climate because the absorption of that carbon dioxide has meant that it isn't around to trap solar radiation and increase Earth's temperature. 

But that relief from rising temperatures comes at a price. 

When carbon dioxide dissolves into water, it makes the water more acidic. So, as the ocean absorbs more carbon dioxide, its pH is decreasing. This change is devastating for ocean ecosystems. 

Scientists estimate that the ocean's pH has declined by 0.1. This might not seem like a lot, but pH is measured on a logarithmic scale. That means that pH 7 is ten times more acidic than pH 8. And unfortunately, a lot of oceanic life relies on seawater being slightly basic to survive. 

Many shell-building creatures, like corals and oysters, make their shells out of calcium carbonate. In seawater that is slightly acidic, they can't gather enough carbonate ions to make their shells. In the more acidic seawater of the future, their shells will start to dissolve, and they will die.

Coral reefs are the rainforests of the ocean. They are incredibly biodiverse places, with about 25 percent of all known oceanic species relying on coral reefs. And they're in trouble. A combination of warming water and ocean acidification means that 99 percent of shallow-water corals are threatened with extinction, with troubling implications for the animals that live there. If we do nothing about climate change, by 2100, our existing coral reefs might simply be gone. 

Ocean acidification has economic as well as environmental effects. Already, current levels of ocean acidification are making it impossible for oyster larvae in the Pacific Northwest to form their shells, and as oceans acidify, this could become a global problem. Some experts project that the total loss to the shellfish industry from ocean acidification could be over $1 billion by 2100. And salmon fishing – an $18 billion industry – is also at risk from ocean acidification. Salmon eat pteropods, a type of marine snail that also makes their shells out of calcium carbonate. As the ocean acidifies, pteropods will have a harder time surviving, affecting everything that eats them. 

Animals that cannot move will go extinct

Such climatic changes are projected to occur all over the world, and will ultimately force species to leave zones that are no longer habitable to them and migrate poleward. On land, this means more non-native species moving into areas they haven't previously lived in, and tropical insect-borne diseases moving into temperate zones. The sudden change in species ranges will reshuffle entire ecosystems, as they adapt to new and invasive species.

That only considers the species that can move. Some species, like trees which migrate extremely slowly, are expected to die out entirely with changing climates, leading to worldwide mass extinction events. 

These are only a few of the effects projected worldwide due to climate change. We are still learning how rising global temperatures can affect every aspect of our lives, from increased risks of wildfire and drought to heavier rainfall in the tropics, to deadly urban heatwaves and changes in crop yield. Climate change is even suspected to have psychological effects, with rising temperatures changing human behavior. As time goes on, it's likely that we will discover more potential effects of climate change. 

Even if we stopped emitting fossil fuels tomorrow, Earth would still warm by 1.1 degrees Celsius. But just because some climate change effects are inevitable does not mean that the outlook for our planet is hopeless: we can still stop the worst effects from happening. If we take serious steps in the next ten years to reduce carbon dioxide emissions, such as switching to renewable sources of energy, making it easier to purchase and drive electric cars, and cutting back on carbon emissions on a global scale, we can reduce the impact of these effects. 

One of climate change's biggest impacts impacts is on some of the smallest members of the Arctic ecosystem – phytoplankton. "Phytoplankton," a collective term for marine organisms that photosynthesize, are barely visible to the naked eye, but these small creatures produce up to half of the world's entire oxygen supply. And, as a recent study by Martí Galí and other researchers at the Université Laval in Quebec shows, they might be having an even stranger impact on Arctic climate. 

Phytoplankton need two things to grow: light, and nutrients. In the Arctic Ocean, nutrients are rarely a problem. The Arctic's long winter nights, however, mean that for most of the year, Arctic phytoplankton do not have enough light to truly grow. The majority of Arctic species lie in wait for enough sunlight to trigger their growth, and when they receive that sunlight, they bloom in abundance. But in order for sunlight to reach the phytoplankton in the ocean, these plankton have to be in a part of the Arctic that isn't covered by ice. The decrease in Arctic sea ice means that regions of the Arctic Ocean are ice-free for longer periods of time, which means that Arctic phytoplankton are getting larger and larger regions in which to grow. This means an overall increase in the amount of Arctic phytoplankton, and when Arctic phytoplankton increase, so too might a certain gas in the atmosphere – DMS. 

DMS stands for dimethylsulfide. At the low concentrations produced by phytoplankton, it's generally harmless. If you've gone to the beach, you've smelled DMS at some point in your life. It's a somewhat disagreeable odor, the same one produced when cooking corn, boiling cabbage, or certain types of seafood. It's a crucial component of the instantly recognizable "smell of the sea." It's produced naturally by certain types of phytoplankton, and so, it's natural to assume that as phytoplankton increase, DMS concentrations increase. But DMS is particularly interesting not just because of its odor, but because of its suspected role in producing clouds. The full extent of the link between DMS and cloud formation is still unknown, but if DMS does affect cloud formation then, Galí and his collaborators suggest, the loss of sea ice in the Arctic might actually be contributing to more Arctic clouds.

Testing whether phytoplankton in the Arctic were producing more DMS than normal wasn't straightforward, because current measurements of DMS in the atmosphere are sparse. And because travel to the Arctic is difficult, expensive, and sometimes dangerous, there are very few actual measurements of the composition of Arctic air, or the chemistry of the water. In order to estimate whether DMS concentrations were actually increasing in the region, Galí and his team had to take a wider perspective. They had to look at the region from space. 

The satellites SeaWIFS and MODIS-Aqua have monitored chlorophyll-a, the pigment that gives plants their green color, in the ocean since 1997. Chlorophyll is often used as a stand-in for phytoplankton biomass – the more chlorophyll is present in the ocean, so the reasoning goes, the more phytoplankton are active. This reasoning has some issues – not all phytoplankton use chlorophyll as their main pigment, for example – but it does a good job at giving us a general idea of whether phytoplankton populations are increasing. 

By combining chlorophyll measurements from the satellites with an algorithm that converts biomass to DMS emissions (by calculating how much DMS is emitted by phytoplankton on average), Galí and his team were able to estimate that DMS concentrations in the Arctic have increased by roughly 33% per decade. Taking these estimates and projecting them into the scenario of the Arctic ocean being completely ice-free in the summer, they estimated that DMS concentrations could more than double in the event of a completely ice-free summer, something projected to occur as soon as the next 25 years if climate change remains unchecked. This means that in the future, Arctic skies could have more than twice the amount of DMS they have today.

 Which brings us back to clouds. Currently, the link between DMS emissions and cloud formation is something we don't fully understand, but we do see that DMS does have some impact on cloud formation. If DMS concentrations in the Arctic continue to increase, it's reasonable to expect a change in cloud patterns in the Arctic. And this is important, because not only do clouds play a crucial role in regulating Earth's climate, clouds and the impact of clouds on Earth's temperature are also one of the least understood aspects of climate change. As far as we understand it, clouds can act to either cool Earth, if sunlight is reflected from the tops of clouds and back into space, or further amplify warming, if heat from the land is reflected off the bottom of clouds and back down onto the planet. 

A number of factors go into whether clouds can warm or cool Earth – the altitude of clouds, how many clouds are present, and how opaque or transparent the clouds are, for example. In general, high, thin clouds tend to have a greater warming effect than a cooling effect, while low, opaque clouds tend to cool the earth more than they warm it. In the Arctic, the effect is slightly different – low clouds over the ocean tend to retain more heat in the area during most of the year. 

So if cloud cover does increase in the Arctic, will the new clouds that form worsen Arctic warming, or help keep the Arctic cool? The fact of the matter is, we still don't know. A previous study observed that when Arctic DMS changes, the new clouds that form in the region have a net cooling effect, but there are many other circumstances in which a warming effect might be feasible (for example, if clouds form over pack ice, and the amount of heat radiating through the clouds overpowers the amount of heat reflected by the clouds). We'd like to hope that an increase in cloud cover acts to slow Arctic warming rather than enhancing it, but in order to know for sure, we would need to put a lot more effort into studying clouds and their effects on global climate.

One thing is clear, though. Climate change has never once been a simple issue. It's important to understand how climate change will affect the Arctic, not only because the Arctic is a vulnerable environment that is home to approximately four million people, but also because the Arctic is one of the first regions that are feeling the effects of climate change, and understanding how the Arctic will be affected helps us prepare for climate change across the entire world.

Jenny Roberts, a postdoctoral researcher at the Alfred Wegener Institute in Bremerhaven, Germany, is working on fossil records like these. Roberts and her collaborators recently published an article claiming that lithium in foraminifera shells can be used to calculate the pH of past oceans. 

Foraminifera, or "forams" for short, are single-celled plankton that live in the ocean. There are two types: planktic forams, which live in the surface ocean, and benthic forams, which live on the sea floor. Both types of forams make their shells out of calcium carbonate, and most often out of calcite. Calcite is a hard mineral (it's chemically similar to limestone), so when forams die, their shells frequently end up fossilized in the sediments. This—and the fact that forams live all over the world—make their shells some of the most valuable tools used by scientists who study past climates.

Generally, researchers try to relate some property of the shell to an environmental condition. Roberts and her lab, for instance, examined the chemical composition of the shells. Calcite is a crystal, which means that its molecules are arranged in what’s called a mineral lattice, a regular structure of calcium atoms connected to each other, like a chain-link fence. When calcite forms, impurities often get caught in the gaps, or end up replacing the calcium atom, in a process known as substitution.

Chemical reactions are often dependent on things like temperature and pH. Because the accumulation of impurities in calcite is, on some level, a chemical reaction—where dissolved materials are being converted into a solid phase—researchers can deduce things like temperature, pH, and the chemistry of the ocean based on the amount of impurities in the shells and how those impurities interact with each other. 

Isotopes, other types of atoms, complicate the picture even further. Atoms are composed of protons, neutrons, and electrons. Normally, an atom has an equal number of protons and electrons, but an atom of the same element can have different numbers of neutrons. For example, the most common kind of carbon atom has 6 protons and 6 neutrons. This form of carbon is sometimes called carbon-12, or ¹²C, named after the sum of the number of protons and neutrons. But a carbon atom can have 6 protons and 7 neutrons. In that case, the atom is named carbon-13, or ¹³C. ¹²C and ¹³C are different isotopes of carbon. Both of these isotopes are stable, meaning they won't decay with time.

Lithium, an element that can be found in forams, has two stable isotopes: lithium-7, the common form, and lithium-6, a rare form. Roberts used lithium isotopes to calculate pH in what's known as a culture study; she and her team grew benthic forams in a lab with seawater at an altered pH. As the seawater became more basic, the amount of lithium-7 in their shells decreased. This correlation was strong enough that the researchers were able to accurately calculate the pH of seawater based on the shells' isotopes. Next, the researchers went back to look at old foram fossils, using their lithium isotopes to look back in time and calculate past ocean acidity. Using sediment core samples and lithium-7, they calculated past ocean pH as far back as 25,000 years ago.

Of course, because biology doesn't always do what we'd expect, there are often road blocks and new questions that pop up along the way. In this case, while Roberts found these isotope correlations could be applied to fossil records, her estimated pH differed from results collected using another method of calculating pH from fossils. That method focused on boron-11, a different element, in shell calcite. Further studies will be needed to understand these differences. 

Furthermore, while 25,000 years sounds like a lot, on a geologic scale, it's really not that long—some of the most dramatic climate changes occurred hundreds of thousands to millions of years ago. New research will likely try to find what caused these observed trends, where different methods of calculating pH can be applied, and what conditions might affect or disrupt these equations.

But overall, it's an exciting glimpse into some of the biggest questions we have about the past. The development of a new proxy for calculating ocean acidity gives us a new tool to study past climate changes—and the effect of those changes on the species and ecosystems that existed at the time. The earth has a lot of stories left to tell.

A recent study published by Joe McConnell, a research professor at the University of Nevada, and his collaborators shows that lead levels in Greenland ice cores can be traced back to one very specific activity — silver mining. Roman mining techniques extracted silver by smelting lead-silver ores, and adding more lead to the resulting mixture to concentrate the silver. The two metals would be separated later in the process, but this activity, which was usually done at the Roman mines of Rio Tinto in southern Spain, was the major source of lead emission into the atmosphere for that time.

And those emissions can still be seen in ice sheets, which build very slowly over time. When ice forms in Greenland and Antarctica, it records the chemistry of the atmosphere at the time of its formation, a snapshot of Earth's history. As ice sheets continue to build and thicken, those snapshots are preserved in time, like Earth's own historical records, and while these chemical records may not be visible to the naked eye, they are not invisible to modern analytical techniques. The chemistry of ice cores is often used to study past climates, but McConnell and his team turned their attention to something a little closer to home, revealing a cycle of expansion, plague, prosperity and war.

The rise and fall of the Roman Empire is captured in the lead emission record, but while the Romans greatly expanded mining operations in the Iberian Peninsula, they weren't the first civilization to mine in that region. The first sustained increases of lead emissions began around 1000 BC, almost a millennia before the rise of the Empire. This coincides with the high point of the Phoenician civilization, who were expanding into the western Mediterranean. Emissions continued between 400-200 BC, as the Carthaginians continued mining in Spain, but the signs of emissions drop in 264 BC, at the start of the First Punic War.

During the final years of the war, as Carthage struggled to pay off the mercenaries they had hired, emissions ramped up, reflecting an increase in silver mining to keep up with this demand. (Carthage eventually lost to the rising Roman Republic.) Emissions dropped again when mining activities were disrupted at the start of the Second Punic War, but rose when the Republic seized mines from Carthage in 206 BC.

For the first few centuries of Roman rule, mining activities were interrupted by various wars in the region. Warfare in Spain kept emissions consistently low between 108 to 92 BC, and emissions dropped even further after 80 BC, due to the Sertorian War, when a coalition of Romans and Iberians, led by the general Quintus Sertorius, fought against a regime established by Sulla, the statesman who established himself as the Republic's first dictator in a century. Emissions continued to drop after the Sertonian War due to regular Lusitanian raids, only recovering in 61 BC, under the leadership of the governor of Spain, a man whose name is almost synonymous with the idea of Rome — Gaius Julius Caesar.

After Caesar’s successful campaign against the Lusitanians, mining activities recovered, and emissions increased for about a decade. Then, civil wars and the end of the Republic put another stop to mining activities, which only recovered after the victory of Octavian (later the Emperor Augustus) in 31 BC. From there, emissions increased again, remaining high until around 160 AD, with the occasional short-term fluctuations. This coincides with what became known as Pax Romana, the longest period of sustained peace in European history up until that point. However, emissions were ultimately disrupted by the Antonine Plague in 165 AD, a pandemic of either smallpox or measles that lead to the death of five million people, killing up to a  third of the population in some areas and devastating the Roman army. Emissions did not recover until 750 AD, well past the twilight of the Empire.

The link between these historical events and silver mining can be seen not just in ice core data, but in the composition of Rome's silver coin — the denarius. Surviving denarii from periods with low emissions tend to have lower silver content than denarii from high-emission periods. The drop in silver mining after the Antonine Plague ultimately led to the shift of Roman currency away from silver and toward a compound that was essentially a gold-copper alloy.

The Roman Empire existed more than 1,500 years ago, at a time before the Industrial Revolution, but we can still see evidence of their activities in ice cores today. We can watch, using chemical traces locked in ice, the collapse of a Republic and the growth of an Empire. We can see evidence left behind by wars and migrations that we think of as events of the distant past. But the world inhabited by the Romans isn't, ultimately, all that different from the world we live in today. There were years of war and years of peace, commerce, trade and tragedy, all conducted by human beings very much like ourselves.

The distance between us and Rome might seem immense, and the distance between us and some of the oldest civilizations in the region might seem even larger still, but it's important to realize that as far as the Earth is concerned, this all happened in the blink of an eye. If we condense all Earth's history down into a single calendar year, the Phoenicians, Julius Caesar, and everyone reading this article all exist together in the last few seconds before midnight on December 31. There is an enormous wealth of information about the Earth's past and humanity's history locked up in the ice, waiting for discovery — even though our story only barely scratches the surface.

Iron fertilization as a geoengineering method was mostly considered in the late 1990s to early 2000s, and as an idea, it came about because of a very fundamental oceanographic question: why do certain areas of the ocean have very low amounts of plankton, even when they have enough nutrients for those plankton to grow?

In most areas of the ocean, phytoplankton, the tiny plant-like creatures that produce most of the oxygen in the ocean (and about half of the oxygen on the planet), need only two things to grow: light and nutrients. But there are some areas of the ocean that contain high levels of nutrients and no phytoplankton, even during times of the year when light conditions are right. These regions, called high-nutrient, low-chlorophyll regions, puzzled oceanographers until 1990, when a scientist named John Martin proposed that iron might be the missing link.

The limits of iron 

Iron is a micronutrient – living things don't need a lot of it, but what they do need, they can’t do without. Places like the Southern Ocean, off the coast of Antarctica, have almost no iron at all. This is because iron is a terrestrial micronutrient, meaning it comes from the land. The most common sources of iron to the ocean are rivers and wind-blown dust from deserts. Antarctica, covered in ice and isolated from other continents, has none of that. Martin proposed that that was why the phytoplankton weren’t growing. To prove it, scientists went out and conducted iron fertilization experiments by artificially adding iron to the ocean and see if the phytoplankton grew. Within a day of adding dissolved iron into the ocean, scientists observed a phytoplankton bloom.

That research was interesting and exciting, but the most exciting part for some people was an interesting link between phytoplankton and carbon dioxide. See, just like plants on land, phytoplankton are photosynthesizers. They use up carbon dioxide to create oxygen, and with so much open, iron-limited space in the Southern Ocean, oceanographic researchers across multiple institutions worldwide started to wonder if, with just enough iron, we could stop climate change completely.

It didn't work out quite that nicely. Artificial iron fertilization was proven to be ineffective at removing carbon from the atmosphere. And over concerns, published in a letter to Science in 2008, that continuing to conduct these studies might harm the natural environment by allowing invasive species to grow or by altering the ecosystem in ways they couldn't predict, the research was effectively halted. (Some continued under the table. In 2012, a US-based entrepreneur named Russ George defied an ocean-dumping moratorium, convincing the Haida Nation to conduct an iron fertilization project off the coast of British Columbia, Canada, to boost salmon populations, with the idea that salmon would feed off of the resulting phytoplankton bloom. According to Nature, scientists have seen no evidence that the scheme worked.)

Since 2008, there have been almost no papers talking about iron in the Southern Ocean affecting the climate. However, new research by Gary Shaffer, a researcher at the University of Magallanes in Punta Arenas, Chile, and Fabrice Lambert, an assistant professor at the Pontifical Catholic University of Chile, suggests that the question of iron isn't fully closed. Their research, which is focused on atmospheric dust, a major source of oceanic iron, suggests that dust, and the iron in it, may have contributed to the onset of the ice ages. And understanding how the Earth has naturally cooled in the past, prior to human intervention, would allow us to better understand how Earth might cool in the future.

Simulating sun and iron

Ice ages are actually regular events. Over the past 800,000 years, we've had eight. To a certain extent, they can be explained by variations in Earth's orbit. However, one curious thing about the past few ice ages is that our records show a decrease of carbon dioxide during them, contributing to global cooling via reflection, where dust particles in the atmosphere reflect incoming sunlight back out into space, and iron fertilization.

And while Antarctica is usually far away from any sources of dust, Shaffer and Lambert's research, which combines records taken from dust measurements in ice cores over the past 300,000 years with temperature measurements calculated from those same ice cores in Greenland and Antarctica, shows that the amount of dust in the atmosphere increased greatly just before the start of the last three ice ages.

While they state that this effect is not strong enough to cause an ice age on its own, the correlation is compelling, and Shaffer and Lambert make a convincing case that dust in the atmosphere might have provided the final push into an ice age. Using computer model simulations, they tested both the effect of reflection of sunlight and iron fertilization, and both appeared significant. Their research suggests that iron fertilization may play a part in cooling the earth after all.

However, that still doesn't mean that iron fertilization is the solution to climate change. Shaffer and Lambert's work showed that iron can contribute to an ice age that's already in development, not that it can start an ice age from scratch. The potential risks of artificial iron fertilization still need to be examined carefully before reopening the idea of using it for geoengineering so that we can fully understand the effects that this technique has on the natural environment and be prepared for any consequences that might result.

The East Coast has been the focus of sea level rise studies in the past because of the gentle rise of the continental slope there. In that part of the country, a small rise in sea level can cover a much larger area, and we know just by looking at these slopes that cities on the East and Gulf Coasts of the United States – Miami, New Orleans, and New York City, for example – will be more heavily impacted by sea level rise than their western counterparts. But while the West's dramatic coastlines have masked the effects of sea level rise up to this point, that doesn’t mean that the West Coast will get off free and clear.

With both of Southern California's biggest cities built around the coast, sea level rise is still a threat, and without good predictions about just how high the sea will rise, civil engineers and city planners are working blind. Predicting sea level rise is a difficult, because we simply have never seen sea level rise at this scale before. We are changing Earth’s climate at a rate unprecedented throughout all of human history.

But not, it turns out, throughout Earth's history. Sea level has risen and fallen on the planet before, we just weren’t around to see it. So when it comes to predicting the effects of modern climate change, scientists often have to act as detectives. We weren’t there to witness these changes. All we can do is look at the evidence left behind, and use it to piece together what happened. That’s where foraminifera come in.

Enter the plankton

Foraminifera, or "forams" for short, are microscopic plankton that live in the ocean. They create their shells out of a hard mineral called calcite, and they are preserved as fossils long after the organism dies. Because these small creatures live everywhere in the ocean, from the surface of the water to the seafloor, they are some of oceanographers’ most valuable tools for understanding past climates. A group of researchers from UCLA, led by postdoc Simona Avnaim-Katav, think that one kind of forams can tell us how sea level changed in the region in the past – and, therefore, what we might expect in the future.

Avnaim-Katav looks at forams that live in salt marshes, coastal areas with a pretty dramatic slope, where sea level changes with the tide. In areas like these, different species of foraminifera tend to live at different elevations. They prefer varying light levels, pressures, and salt content, all of which can change with water depth.

In order to understand sea level changes in the region, Avnaim-Katav and her team went out to two salt marshes relatively untouched by human development, one at Seal Beach, California, and the other in the Tijuana River Estuary, in San Diego County. They collected 57 surface sediment samples from different elevations along the coast and looked through each of them to determine which species were living at which elevations.

Census of the sea

From that data, they created what’s known as a species assemblage – a census, basically. They took notes of water depth, temperature, salinity and oxygen, along with a list of which species were living at which sites. Their goal was to create a transfer function, a computer algorithm that could be used to predict water depth based on the species that lived there.

It works something like this: if Species A and B like shallow water, and species B and C prefer deep water, we can predict that a sediment sample from 20,000 years ago that contains species B and C must have been in deep water, even if it’s found in shallow water now. This is a common technique for determining sea level changes, and has been used in multiple sites all around the world, but never before in Southern California. In order to create this transfer function, the biggest question that Avnaim-Katav had to answer was whether the forams there were impacted by elevation at all.

Tijuana to Seal Beach

To do this, they had to turn to statistics. They used an impressive collection of statistical tests to see if elevation was the strongest predictor of species assemblage, or whether it was something else, like salinity, pH, temperature, or oxygen concentrations.

At Tijuana, they found what they were hoping to see. Elevation explained 43.2 percent of the differences between species, with the other parameters sharing the remaining 60 percent. At Seal Beach, however, the story was a little different. Elevation in that region only explains about 16 percent of the species differences, with oxygen being the parameter that most affects where species live. However, when the results from the two beaches are combined, elevation comes out on top, allowing them to produce a transfer function that, while not quite perfect, can give us much more information than we currently have.

And as far as this research goes, this is just the beginning. Avnaim-Katav and her team haven’t figured out how sea level has changed, and they still don’t know how drastically the sea level in Southern California will change in the future. But they’ve figured out how to answer those questions. They’ve developed the pathway that will allow them or future researchers to tackle this problem, bringing us one step closer to adapting to sea level change.