14 April 2023

Coed Olaf - Climate and Climate Change

Having discussed the history of Coed Olaf, we now move onto the nitty-gritty of its environment, so here we discuss its current climate and the likely impact of climate change.

Coed Olaf - Climate and Climate Change

The main factors that effect the nature of the wood, Coed Olaf, and its habitats are physical and chemical. These are weather/ climate, the soil, water and topography. As well as general inputs from UK databases, we are carrying out various scientific surveys ourselves to learn about these inputs so that we can get to grips with how the wood may pan out in the future and to ensure we do not make stupid mistakes in what we try to do with the site.

About a year ago, for example, we installed a weather station, which takes automatic readings of the weather in the woodland, plus we get additional data from the level logger kit that we installed to understand the hydrology of the site. These will enable us to drill into the data that we have got from the Met Office and, if the data is maintained over a period of time, see what the local impacts are from climate change.

The Climate Now

Garnant has a maritime climate, with weather that is often cloudy and wet but mild.

The mean annual temperature is about 10°C (4.4 – 16.3°C), with a soil temperature of 10.6°C (5 – 16.8°C). Temperature varies by season and time of the day. In winter, temperatures are influenced by those of the surface of the surrounding sea, which are lowest in late February or early March, but there is little difference between January and February. Minimum temperatures are usually around sunrise, with the coldest nights being those when there is little wind, skies are clear, and there is a covering of snow. July is normally the warmest month, with mean daily maximum temperatures about 20 °C. Daily maximum temperatures usually occur 2 or 3 hours after midday.

The hilly terrain and Coed Olaf's closeness to the Atlantic encourage cloudy weather, and a haar often hangs over the wood in the morning that paints the many cobwebs with droplets of water. The valley is quite dull with an average annual sunshine total of just over 1,400 hours, in line with the average for Wales but somewhat lower than the 1,750 hours on the south coast of Pembrokeshire. Mean monthly sunshine totals reach a maximum in May and are lowest in December. The key factor is the variation in the length of the day through the year, but cloud cover also plays a part.

It is quite wet, with an average annual rainfall of nearly 1,700 mm, higher than the average of around 1,300 mm for South Wales but much less than the 3,000 mm that Snowdonia gets. October – January are much wetter months than February – September. This seasonality is caused by the high frequency of winter Atlantic depressions and the relatively low number of summer thunderstorms.

Garnant is relatively windy but much less so than along the coast. The annual average windspeed is around 9 mph (7.7 – 11.1 mph). The strongest winds are linked to the passage of deep areas of low pressure close to or across the UK. The frequency and strength of these depressions is greatest in the winter, especially November – February, which is when mean wind speeds and gusts are strongest.

Overall, the climate is oceanic and capable of supporting a temperate rainforest. Average rainfall is about 1,675 mm and so above 1,400 mm per annum, summer rainfall of c. 295 mm or 17% of the total is over the 10% threshold, while the average temperature in July is 16.3°C which is slightly above the 16°C screen for a temperate rainforest. Maps show the wood is located in the temperate rainforest climatic zone, with hygrothermy in the range 125 – 150, which is classified as ‘oceanic’, and lies very close to land to the east that is ‘hyperoceanic’, i.e., the Brecon Beacons.

Climate Change

Estimates for climate change in Wales and Coed Olaf in particular, based on medium-high scenarios in comparison to the 1961 – 1990 baseline, are:

  • A 2.3 – 2.6°C increase in mean annual temperature by the 2050s and 3.3 – 4.1°C for the 2080s. There are similar levels of increase in the winter and summer temperatures except that they are relatively higher in the summer.
  • The wood’s temperature may be 12 – 13°C by the 2050s and 13 – 14°C by the 2080s.
  • The accumulated annual temperature for the wood may be about 50% higher in the 2050s and about 66% higher in the 2080s becoming ‘very warm’ by the 2050s, compared to the current ‘warm’ rating for accumulated temperature.
  • Whereas the overall levels of rain will be unchanged, there will be greater seasonality, with the winters wetter and the summers drier. The mean change may be 13 – 14% in winter and -17% for the summers in the 2050s, and a 19 – 26% increase in the winters and a -20 to -26% decrease for the summers in the 2080s.
  • The moisture deficit of the woodland may worsen by 20% by the 2050s and by nearly 50% for the 2080s, but the ground may remain ‘moist’ although it could become only just ‘slightly dry’ by 2080.
  • There is no predicted change in average or extreme wind speeds, and there is no change in the windiness rating from ‘moderately exposed’.

Overall, the wood’s climate in 2080 may be like mid-Glamorgan’s in 2022, so for somewhere between Port Talbot and Pontypridd.

Generally, this means that (from a climatic point of view) the trees that are currently here should still be appropriate in the future, but that beech may become happier to grow here in the future as the climate becomes warmer, whereas currently Garnant is beyond beech's natural zone. The core of the trees now is a sessile oak woodland with downy birch, grey and goat willow, hazel and rowan. We don't expect this to change much over the next 250+ years.