Category: Ask a Climate Scientist
You’ve got questions, and we have answers. No question is too simple or complex for our panel of science advisors, who stand ready to field your questions about climate change.
This week, Nadir Jevanjee answers a question we received about CO2 emissions. Nadir is a Research Physical Scientist at NOAA’s Geophysical Fluid Dynamics Laboratory.
Question: There is evidence that as the oceans warm they release CO2. If CO2 is a major cause of global warming then this represents a positive feedback cycle. If that is the case then why, in the past (over millions of years), have there been times of higher global temperatures and higher CO2 levels that have not resulted in an ever increasing global temperature? What turned off the ‘switch’ in the past? If anthropogenic CO2 is about 5% of all CO2 emissions and if CO2 is about 5% of all greenhouse gases (GHG) – water vapor being 95% – then anthropogenic CO2 represents about 0.25% of total GHG. How is it that elimination of such a small fraction of GHG will reverse global temperature changes.
Answer: Sounds like there are two separate questions here. The first is about a ‘carbon-climate feedback,’ in which increasing temperatures drive CO2 out of the ocean, which should then contribute to further warming. This is indeed a positive feedback, as the questioner points out, but positive feedbacks do not always amplify forever; in most cases, actually, the amplification peters out. This means at some point in the future, ocean temperatures will stabilize at a new equilibrium, but at a much warmer point than today. Unfortunately, it will not happen soon enough to prevent temperature increases that are inhospitable to our species.
As for anthropogenic CO2 being a small fraction of CO2 emissions and all GHGs – that probably stems from a confusion between gross and net fluxes of carbon into the atmosphere. As can be seen in Fig. 3 here, there are vast exchanges of carbon between the atmosphere and both the oceans and land biosphere (the gross fluxes), but in a preindustrial climate these net out to zero (zero net flux). Put more simply, the greenhouse gases emitted by the land and ocean get reabsorbed, keeping earth’s natural reservoirs in balance.
This is important. Before humankind started using fossil fuels, GHG levels stayed relatively stable for 10,000 years and helped set and maintain the earth’s temperature at a level that enabled our species to thrive and grow. The GHG that we are adding by burning fossil fuels, while seeming small in the overall picture (gross fluxes), is enough to upset that balance. Small amounts of greenhouse gasses play an outsized role in impacting our climate – they comprise less than .04% of our atmosphere yet prevent our planet from being a frozen uninhabitable wasteland – so adding to them even a relatively small amount has already led to big changes in climate systems.
Also, it is important to remember that climate, like the human body, is impacted by many influences. For example, as the questioner points out, warmer oceans (all else being equal) hold less CO2, just as warm soda goes flat much faster. Yet, the ocean CO2 levels are rising, despite the ocean’s warmer temperatures. This is because the dominant influence on oceanic CO2 levels is the CO2 concentration in the atmosphere, which must be in balance with the CO2 in the ocean. As atmospheric CO2 levels rise, there is more CO2 entering the ocean from the atmosphere than exiting it, so the oceanic CO2 levels must rise until a new balance is achieved.
The latest Q&A with our climate science advisors explores hurricane forecasts, the difference between water cycle and water table, whether we could be headed toward a glacial period, and more.
In January we presented the C-Change Primer to a California land trust made up of liberals and conservatives who were so divided on climate change they could not talk about it among themselves. C-Change was asked to provide a common set of facts to lay the groundwork for a discussion about the impact that a warming planet is having on the land entrusted to their care. It was such a gratifying experience.
Several skeptics in the audience asked climate questions that were better suited for C-Change’s three science advisors. About that time, we also received an email from a man in the small village of Bagshot, in Surrey, England, asking other science-related questions about climate change. Today, we are pleased to share this Q&A from California to England!
Through non-partisan education and outreach, C-Change Conversations aims to improve understanding about the science and effects of climate change and encourage ongoing discussion and engagement. We welcome your questions from across the globe and right down the street.
The C-Change Conversations Team
Sea Level Rise
Considering the anomalous expansion of water at or about freezing point, why would melting sea ice glaciers cause a rise in sea levels?
There is no such thing as a “sea ice glacier.” Sea ice doesn’t contribute to sea level rise, but glaciers do.
This is a common point of confusion, however, and the question raises an important distinction. Sea ice, which floats, does not raise sea level when it melts. The mass of water in a block of sea ice is exactly equal to the mass of water the block displaces, a fact that goes back to Archimedes. The volume of the ice is slightly larger, however, which is why a little bit of ice (the proverbial “tip of the iceberg”) pokes out above the water. All this means that melting sea ice in the Arctic Ocean does not contribute to sea level rise.
Glaciers, however, occur on land and do not float. Glacier melt thus adds water to the ocean, so the melting of glaciers on the landmasses of Greenland and Antarctica do cause sea level rise.
Am I safe to assume most of the glacier melt consists of fresh water that has a lower density than sea water and has an effect upon deep ocean currents?
Yes! Glacier melt is fresh and thus less dense than sea water, as it has no dissolved salt to weigh it down.
And am I right to assume that a large volume of fresh water entering the deep ocean currents could have a significant long-term effect upon climate and weather regulation?
Right again. Climate scientists have probed exactly these effects in computer models, and indeed large additions of fresh water have been shown to impact the global ocean circulation and hence global climate. These effects were dramatized (rather inaccurately!) in the 2004 film The Day after Tomorrow.
The Fundamentals of Rising Temperatures
Liz Sikes, another C-Change science advisor, helped us answer a variety of questions from a California land trust.
If the world did become carbon neutral or even negative, is there any possibility that the environment would “autocorrect” the earth’s temperature by bringing it down to pre-spike levels?
Short answer: The system can’t autocorrect quickly enough to matter in the next few centuries. It will require some sort of CO2 “capture” to bring temperatures down in a realistic human life-span timeframe.
Long answer: The earth’s system (biosphere, geosphere, hydrosphere) can autocorrect and can control the partitioning of carbon and CO2 between the atmosphere, land, ocean, etc. But this is a slow process. The earth’s system took approximately 10,000 years to release roughly 80 ppm of CO2 from the ocean at the end of the last Ice Age. This was a very, very quick change on natural timescales. We’ve added about 120 ppm to the atmosphere in about 150 years. It will take technical help to reverse that quickly.
Can you provide reference sites to graphs that show the temperature rising?
How do we know what the temperatures were 10,000 years ago?
Short answer: We have proxies that preserve a temperature record. We can read the proxy record of temperature to know what the temperature was. For example, tree rings are proxies. Trees react to temperature and moisture in their environment, and their annual tree ring width can be measured and interpreted to “read” the record of temperature and moisture.
Long answer: There are several proxies for past temperature that are preserved in what we would call the geologic archive. A proxy in this context is something that varies with temperature and then preserves that temperature as time passes.
One of the most widely used temperature proxies is “oxygen isotopes.” Temperature influences the isotopic content of water across the planet, and we can “read” the changes in isotopes like oxygen-18 content in ice cores, marine sediments, and stalactites. The lighter, lower boiling isotope evaporates more quickly and is separated and transported away as a function of temperature.
There are many, many proxies for temperature because so many living organisms respond to temperature, and so many reactions are controlled by temperature. You just need to know how to read the signals.
Depending on the proxy and the “archive” it is in, we can have temperature records that can give more or less detail. Generally, a fast growing archive such as corals gives more detail. Corals can provide temperature records that can give approximately monthly resolution but corals only live decades to a few hundred years, so the records are short. A slow growing archive, like a glacier, from which we can drill ice cores, can give a record of many hundreds to many hundreds of thousand years, but the resolution in those are at best a decade to a few hundred years. Tree ring records tend to be intermediate with annual resolution and a few hundred to a few thousand years long. So high resolution records tend to be short and low resolution records long. Generally no archive can record daily temperatures—but a monthly average is pretty powerful, as we are trying to understand past climate, which is the overall average of weather.
All proxies react to more than just temperature. Verifying and calibrating is important. This is why there are error bars on any measurement, and there are sometimes differences in the record from different proxies. That is why we have so many; we work hard to read the preponderance of evidence for an accurate record.
Climate Change & Tornadoes
Bernadette Woods Placky is our go-to science advisor for questions like this one.
We hear a lot about climate change being linked to hurricanes and wildfires, but what about tornadoes—like the one in Alabama in January and the derecho in Iowa last year? Do we know if climate change plays a role?
While we’ve learned something about how climate change is affecting tornadoes, there is still a lot to learn.
For tornadoes to form, you need three key ingredients:
1) a warm, moist air mass
2) something to trigger thunderstorm formation (a front, dry line, etc.)
3) shear, which is the changing of wind speeds and direction with height upward in the atmosphere.
In a warming world, we know that heat and moisture are increasing. However, there are still questions about how climate change is changing our jetstream, which sets up our weather patterns that, in turn, create shear in the atmosphere. If we don’t have shear, we don’t get the needed spin for tornadoes. Also, the tornado reporting record has changed over time, so we don’t have as long and consistent of a tornado record as we do for other events.
What we have learned so far, though, is that our warming planet seems to be changing how we experience tornadoes. When we do have tornado “outbreak days” (days when there are multiple tornadoes forming), there seem to be more tornadoes striking on those days. So the outbreak days may be getting bigger when they do happen. And there has been some evidence that the main geographic area for tornadoes may be migrating farther east—into what we call Dixie Alley—as opposed to the traditionally recognized Tornado Alley. A warming climate may be responsible for both bigger outbreak days and the shifting geography of tornadoes.
My impression is that this “3%” generally consists of scientists who acknowledge that CO₂ is rising due to human activities, and who also acknowledge that this rising CO₂ will cause some global warming, but who disagree on the amount of warming it will cause.