YOUR QUESTIONS ANSWERED
What is the impact of the ozone layer on climate change? Is the ozone layer closing all that troubling?
Nadir: Ozone is an atmospheric gas made of three oxygen atoms bound together (chemical formula O3). It constitutes a very small fraction of the air close to the Earth's surface which we breathe in and out every day. Most ozone is instead found high up in the atmosphere -- even higher than the level at which most airplanes fly ! This large concentration of O3 in the upper atmosphere is what we call the "ozone layer."
Pollution from human activities (especially CFCs) breaks down the ozone layer, a phenomenon known as the "ozone hole" which was observed in the 1970s . This was a concern as the ozone layer regulates the amount of harmful UV radiation reaching the Earth's surface. This concern led to the 1989 Montreal Protocol banning CFCs and other "ozone-depleting substances." As a result, the ozone layer is now recovering .
How does this connect to climate change? It turns out that ozone not only absorbs UV but is also a greenhouse gas in its own right. The "ozone hole" thus decreased ozone's greenhouse effect, and its current recovery then increases it back, yielding a positive radiative forcing (i.e., change in the O3 greenhouse effect) at present relative to recent decades. But this effect is small, amounting to only a fraction of the radiative forcing from human-emitted CO2 . Thus, the closing of the ozone hole and recovery of the ozone layer should not be seen as a significant driver of climate change.
 Langematz, U. et al. "Scientific Assessment of Ozone Depletion: 2018." Global Ozone Research and Monitoring Project Report 58 (2018).
 Ramaswamy, Venkatachalam, et al. "Radiative forcing of climate: the historical evolution of the radiative forcing concept, the forcing agents and their quantification, and applications." Meteorological Monographs 59 (2018): 14-1.
The earth’s temperature has fluctuated since the beginning of time. How do we know this isn’t just another warming phase?
Nadir: Natural fluctuations are indeed a feature of Earth’s climate. But, if we look at the climate of the past 150 years (for which we have direct temperature measurements) and compare to climate models, we find that the observed warming of the past 30 years or so is outside the range of natural fluctuations produced by models. Furthermore, models can only reproduce this warming when forced with the historically observed CO2 increases (see Fig.1 of FAQ 10.1 of the last IPCC report).
Models are not perfect, of course, so we’d like to have another line of evidence which doesn’t rely on them. One such line of evidence is that warming from natural fluctuations would draw heat from the deep ocean and into the atmosphere, but we instead find that the deep ocean is warming at the same time as the atmosphere (see Fig. 2 of Cheng et al. 2017 below). For people who want to delve more deeply into this topic, I recommend FAQ 10.1 of Ch. 10 of IPCC 2013 and Climate Change: Evidence and Causes, NAS, Ch.2: https://www.nap.edu/read/25733/chapter/5.
Fig. 2. Changes in ocean heat content (OHC), global mean surface temperature (GMST), and sea level rise (SLR) during the past decade. All values are 2-month means; the dashed red lines indicate linear trends. The scale of the y axis is adjusted so that the linear trend has exactly the same slope for all three indices. El Niño events are marked as pale red bars, and the La Niña events are pale blue bars. All time series are referenced to a 2004–2015 mean. The OHC, GMST, and sea level data reported are archived.
How do we know the fires in Australia and storms like Hurricane Sandy were worsened by climate change?
Bernadette: Science has advanced to the point where we can analyze an extreme event and tease out the role that climate change played in that event—how much more/less likely, how much stronger/weaker. A National Academies study reviews how this emerging science has advanced; you can read it here.
Regarding the fires, the World Weather Attribution team did a study on the recent Australian fires and researched the conditions that would support this kind of fire, through a Fire Weather Index. You can read the full study here. Here's a summary of their findings:
Four climate models for which FWI (fire weather index) could be calculated show that the probability of a FWI this high has increased by at least 30% since 1900 as a result of anthropogenic climate change. As the trend in extreme heat is one of the main factors behind this increase and the models underestimate the observed trend in heat, the real increase could be much higher. This is also reflected by a larger trend in the Fire Weather Index in the observations.
With hurricanes, it's hard to detect at this point if climate change made Sandy stronger, but there is a lot that we know about climate change and hurricanes. For example:
A warmer atmosphere leads to more rain, so there is more heavy rainfall with landfalling storms.
Rising seas are creating higher storm surges that go farther inland.
Warmer water is intensifying storms faster.
On our current emissions path, experts predict 1000 parts of carbon dioxide (CO2) per million in the atmosphere by the end of this century. Have levels ever been that high before? If so, when was that and what was the earth like then, especially with regard to sea levels and temperature?
Liz: The last time the atmospheric CO2 was above 1000 ppm was probably the Triassic, more than 200 million years ago. Atmospheric CO2 has varied in the past. It was as high as 4000 ppm in the Cambrian (500 million years ago), before vertebrates evolved. This was a period of arthropods like Trilobites. Both periods are believed to have been warmer than today by several degrees Celsius.
The Triassic (220-200 million years ago) predates the Jurassic, famous for its dinosaurs. It was the time of reptiles and early dinosaurs and when the very first mammals evolved. The continents were joined into a single mega continent called Pangea. The Triassic continental climate was generally hot and dry and there is no evidence for glaciers at the poles. Instead it appears that the poles were moist, temperate, and forested. This is long enough ago that relative sea level is hard to estimate, but it was likely higher.
Oceans play a big part in moderating our climate. Do we have any reason to think that will change?
Liz: The short answer? No. But that doesn’t mean there is no need for worry.
There are two important ways the ocean controls climate: through the heat that the ocean holds and the gas exchange with the atmosphere. The ocean (and water in general) has a higher heat capacity than air or land. Practically, what that means is that most of the heat the Earth gets from the sun goes into the ocean, and once it is there it is less easily released. So how warm the ocean is controls how warm the air and land is overall, although this varies locally a lot. For example, we go to the beach in the summer because it’s cooler; the ocean is still cool from last winter. In the winter it’s warmer at the shore because the water is still relatively warm from last summer; that’s why beaches get less snow. Almost 90% of the warming of the Earth in the last 50 years has been in the ocean. This means without that uptake air temperatures would be much higher. There are also other consequences. As water warms, it expands, and this thermal expansion is contributing measurably to sea level rise which is accelerating.
Ocean-atmosphere gas exchange is a bit more complicated in detail, but for climate, the important gas is carbon dioxide (CO2). The ocean holds 50 times more CO2 than the atmosphere and how much depends on ocean dynamics (circulation, photosynthesis, winds and much more). The atmosphere’s CO2 content is increasing due to human input. To date, nearly half of the anthropogenic CO2 that we have put into the atmosphere has been absorbed by the ocean. Without this uptake by the ocean the CO2 content of the atmosphere would be much higher and again, the earth would be much warmer. But there are negative consequences here too. A warmer ocean can absorb less gas and releases it more readily, particularly CO2, just like a warm soda de-fizzes faster and holds less fizz. Changes in circulation as the ocean warms, can make this worse. Another important consequence of increasing CO2 levels in the ocean is that CO2 is mildly acidic and so by taking up so much CO2 the ocean is becoming more acidic. This Ocean Acidification is threatening organisms like coral and oysters that need calcium carbonate shells and have skeletons that dissolve under acidic conditions.
For more information on ocean and heat go to: https://www.climate.gov/news-features/understanding-climate/climate-change-ocean-heat-content
For more information on ocean CO2 uptake go to: https://www.pmel.noaa.gov/co2/
Source: National Oceanic and Atmospheric Administration