The Year of Hot Water

The Year of Hot Water

NOAA on 2020 ocean temperatures

Sailors, 2020 was a lot of things – we were disrupted and shaken-up. Things changed, for our societies and importantly, for our environments. Swathes of our planet may have rested and flourished; with air, shipping and tourism noise and pollution dropping to an unprecedented low, but 2020 was also a very hot year. The joint hottest on record, in fact. 

It was also the hottest year on record, for our oceans. And a record-breaker for the number of hurricanes.

Clean Sailors’ Lead, Holly, takes a closer look…

Given the size and depth of our seas, it can be difficult to wrap our heads around the changes that they are undergoing, particularly over the last ~50 years. Outwardly, and aside from the plastic litter we sometimes see bobbing in them, the appearance of our oceans has largely remained the same; still big, still blue, still with regular tides, currents, swells, still supporting marine life living in them and still great for sailing. It is only by looking closely and across a whole range of measurements and underwaters studies that the true picture of seas becomes clearer.

One thing that has been discovered by scientists over and over, is that our seas are getting warmer. Now, given the sheer size and depth of our waters, which make up 70% of our whole planet, running up to 7 miles deep, getting this body of water up to record temperature is quite something. Something we need to take seriously.

Let’s take a quick look at how our oceans work, before understanding why and how warmer oceans impact us and what it could mean for us sailors out on the water.

Here's how the ocean works...

  1. There are two types of water in our seas

Salt water and fresh water.

Salt water is heavier and denser than fresh water, so it sinks. Salt water also has a lower freezing point than fresh water, so it can get pretty cold before it turns to ice. This is why you often see the top layer of sea water freezing, but not all the way through.

Deep, cold, salty currents make up the lower layers of our oceans and move slower than their warmer, freshwater counterparts. When sea ice freezes, it leaves salt behind, making denser, saltier water which sinks and helps to generate the conveyor belt of our oceans.

In contrast, fresh water is lighter and less dense, plus it has a higher freezing point, so when it enters the ocean (through rain or through rivers) is stays on the surface as a warmer layer of water.

Fresher, less salty water makes up the surface currents of our seas and move faster than the deeper, colder, saltier waters.

  1. Heated by the Sun, the ocean creates a conveyor belt

Each day, our planet is warmed all the way around by the Sun; some areas more than others and at different intensities, depending on the seasons.

As our waters get heated, they expand, driving currents around our planet, from the hottest areas (Persian Gulf) to the coldest (of the Arctic Ocean). Crudely, warmer waters move towards the poles, where they get cold and sink, returning back towards the equator.

The circulation of our ocean also gets helped along by the push and pull of tides, plus surface winds skimming over the uppermost layer.

Our oceans getting warmer and cooling off again, in the evenings, due to seasons and at lower altitudes (as you get closer to the poles), exchanges heat into the air above it, helping to create our global weather systems, too.


How ocean currents work


  1. Adding heat and more freshwater changes the ocean dynamic

Year-round, fresh waters are added to the saltier waters of the ocean. Whether from rainwater, snow/ice melts or water freezing, all affect the temperature and therefore currents of our seas. Over the course of the year, the Sun’s strength on our planet fluctuates with the seasons, all of which impacts how fast our ocean currents move, and where to.

You can see, then, how increasing temperatures melt glaciers and ice sheets, adding warmer fresher water to our oceans, which then drives an impact on our whole global environment from sea behaviour to our weather systems. Most of this happens on a seasonal basis throughout the history of our planet, but it’s getting faster and more pronounced as a result of global warming.

Why are our seas getting warmer?

Good question.

Our oceans absorb heat – always have done. They soak up the heat of the Sun, which then drives currents and our weather systems, but over the last years, there has been a continued increase in our global temperatures due to greenhouse gases and other impacts by us humans.

We have excess heat in our global system, of which ~90% is absorbed by our seas.

Excess heat in our global system = absorbed by our oceans = record-breaking sea temperatures.

What impact will warmer seas have on us?

  • When things get warmer, they expand

Just like getting a ring off your finger on a hot day, virtually everything expands when it gets warmer. The sea is no exception.

Warmer seas = expanding seas

The impact? More frequent and greater coastal flooding, greater erosion activity. With over one third of the total world population living near a coastline, the results of continued warming and expanding seas could be devastating. Think, too, of all the marinas and sea walls currently providing protection to coastlines, towns and our boats.

  • Warmer seas fuel super-storms

It’s no coincidence that as well as 2020 being the year of the warmest seas, we also saw the greatest number of Atlantic hurricanes recorded – 30 named storms in total.

The most violent on earth, these storms use warm moist air as fuel, and only form over warm ocean waters near the equator.


  • Storm zones and seasons will increase

As we are already seeing, more heat in the ocean is highly likely to increase the number of hurricanes and cyclones forming, and if warmer waters because more widespread, we could see these storms getting stronger, forming in higher latitudes, further away from the equator than usual and extending the length of the season in which they occur. The current season of cross-ocean weather systems gets disrupted, changing what weather arrives when and how strong weather events actually are.


Nasa image of hurricane

  • Marine life (and our food chain) gets disrupted

As most marine species live in the top, surface layer of the ocean they are exposed to where temperature changes happen most frequently, and the quickest.

Our food chain, marine species included, is based on a delicate balance– just the right temperature, salinity and chemical conditions at just the right time, for mating, spawning, hatching and indeed for fishing. Warmer ocean regions getting warmer and colder regions getting colder will continue to disrupt the status quo for our marine species.

Cold, nutrient-rich waters need to be pushed up to warmer water delivering nutrients to seaweed and plankton, the diet for almost the entire marine food chain. Responsible for producing oxygen, phytoplankton have been shown to grow slower in periods of warmer seas. Studies have also already shown that some fish species halve in number, as a result of marine heatwaves.

An increase in sea temperatures also results in coral-bleaching; huge swathes of our shallow waters which provide breeding grounds and nurseries for a huge portion of marine life globally.

  • New species seen in waters around the world

Not as positive as it sounds! Think jellyfish…

Should seas continue to warm, and warm areas spread, we will likely see warm water species appearing in more parts of our oceans. To date, an important marker of sea temperature increases has been jellyfish. Jellyfish like warmer waters and over the years have been regularly spreading further North – they are now commonplace in southern parts of the UK, for example, and a lot more frequent in the Mediterranean.

Although incredible species, jellyfish are only are they a pain for swimming but impact the food of other species including plankton and fish. This in turn, impacts our food chain…

  • Storing more heat means less room for storing carbon

Global warming aside, there are lots of marine species, most plankton, full of organic carbon. These microscopic plants soak up carbon-dioxide from the atmosphere, converting it into organic carbon. When these species die, they sink to the ocean floor. Studies show that when the water is warmer, however, the plankton dissolve at the surface, rather than deep in the ocean, keeping the converted carbon at surface level.

The result? The carbon can readily return into the atmosphere as carbon dioxide, where is it already an overly abundant greenhouse gas...

What could warmer seas mean for sailing?

Well, Sailors, there are couple of things that will likely change for us if sea temperatures continue to rise.

  • Hurricane season could be stronger and last longer, changing our path and timings for Atlantic crossings and offshore sailing, in particular. As above, storms would get stormier, with heavier rainfall and stronger winds, and be potentially more widespread.
  • Should sea-levels rise, our safe harbours may not be, well, so safe. More violent storms meaning out coastlines and increased flooding around coastal ecosystems could change where and how we keep our boats, and when.
  • As we’ve seen the state of our oceans impacts our weather systems directly. Sustained changes to the temperature of our oceans is likely to have an impact of our weather systems and potentially wind patterns.
  • With warmer sea temperature comes the spread of invasive species that love warm water. It could be that we see newer, warm-water friendly species living on our hulls.

Barnacles have been shown to thrive in warmer waters, of around at least ~15 degrees Celsius. Warmer sea temperatures for more regions of our planet could see an increase in barnacles and other species on the hulls of our boats. And you know what this means…more bottom-scrubbing and better anti-fouling tactics, needed!