Deadly Threat No. 1: It’s getting hot in here…

As I mentioned in my last post, anthropogenic forcing has caused atmospheric concentrations of carbon dioxide (CO₂) to increase. When observations began in 1958, CO₂ levels were approximately 315 ppm, but today, they have reached 393 ppm (CO2 Now, 2013). This increase in CO₂, alongside other greenhouse gases such as methane and nitrous oxide, has changed the radiation balance of the Earth. Now, more radiation is received than emitted at the Earth’s surface and consequently, we are seeing a rise in global average air temperatures (Tyrrell, 2011). Indeed, the IPCC report from Working Group 1, published last month, declared that ‘each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850’. 

However, not only has the atmosphere warmed, it has induced ocean warming. This is what Bijma et al. identifies as the first deadly threat (2013). With a heat capacity 1000 times that of the atmosphere, the ocean has become the primary heat sink for the planet. Between 1971 and 2010, ocean warming accounted for over 90% of the extra energy added to the climate system (IPCC, 2013). However, thanks to the ocean’s thermal inertia, a time lag exists in the global warming response to elevated greenhouse gases (Pierce et al., 2011). To explain this simply, imagine you are heating a saucepan of water. Even though the flame (atmosphere) has a temperature in the hundreds of degrees celcius, the water (ocean) takes time to warm up and boil. This means our children and our children's children, are already committed to climate change associated with the current radiation imbalance.

The latest statistics from the IPCC show that between 1971 and 2010, the upper 75m of the global ocean warmed by 0.11 [0.09 to 0.13] ^oC, per decade. The degree of warming decreased with depth, reducing to less than 0.02^oC per decade, at 500m (IPCC, 2013). This ocean warming has had and will continue to have a whole host of implications (see bold below). However, as they are all big issues in their own right, I do not feel like I can do every one justice in this blog post. Therefore, I will briefly introduce them, with the intention to cover each in greater detail in the future!

Impacts of Ocean Warming

Sea level rise - With an increase in temperature, seawater expands (a process called thermal expansion) and this causes a rise in sea level. Simultaneously, the warming of higher latitudes promotes the melting of polar and land-based ice, adding further water to the ocean (Tyrrell, 2011). Together, these increase sea level and pose various threats, such as: coastal inundation, shoreline erosion and more powerful storm surges. From 1901 to 2010, the IPCC report stated that sea level has risen by 0.19 [0.17-0.21]m, at a much larger rate than the mean over the previous two millennia (2013).

Changes to the thermohaline circulation – The thermohaline circulation describes the movement of water driven by variations in density (Barry and Chorley, 2010). As ocean temperatures increase, surface waters become less dense. This increases their buoyancy and, subsequently, inhibits sinking. Similarly, with the melting of polar ice, low-salinity water is added to starting points of the thermohaline circulation (locations where surface water normally sinks to great depth). This lowers the salinity of surface waters, decreasing their density, which again inhibits sinking. Together, these both lead to increased stratification of ocean layers. Lighter, warmer waters float on top of the denser, colder waters, preventing vertical mixing (Tyrrell, 2011). This has repercussions for both the global transport of heat and nutrient supply. 

Decreased surface oxygen concentrations – Oxygen’s solubility decreases in warmer water and, therefore, with ocean temperatures increasing, there is less oxygen in the surface seawater. This deoxygenation has many impacts on ocean biodiversity and as the third deadly threat, will be discussed in depth in a couple of weeks, so stay tuned.

Stronger storms – There are two factors contributing to this. Firstly, ocean warming is providing many more opportunities for hurricane formation. Hurricanes require a minimum sea surface temperature (SST) of about 26^oC and this is becoming more common as SSTs increase. Secondly, with the ocean warming, it's excess heat is being expelled through evaporation, further warming the atmosphere. This increases the air's water vapour capacity meaning there is more fuel for potentially larger storms (Trenberth, 2007).

So, it's clear that the warming of our ocean is having a multitude of impacts. Some of you may be wondering why I haven't discussed how these are affecting the ocean's marine life. Don't fret - I will be focusing on this next time!