YOUR QUESTIONS ANSWERED
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.
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