Climate change very likely has effects on vegetation so that trees grow faster due to carbon dioxide fertilization (a higher partial pressure increases the rate of reactions with Rubisco during photosynthesis) and that trees can be established in new territories in a warmer climate. This has far-reaching significance for the climate system mainly due to a number of feedback mechanisms still under debate. By simulating the vegetation using the Lund-Potsdam-Jena guess dynamic vegetation model, a territory in northern Russia is studied during three different climate protocols assuming a doubling of carbon dioxide levels compared to the year 1975. A back of the envelope calculation is made for the subsequent increased levels of emissions of monoterpenes from spruce and pine forests. The results show that the emissions of monoterpenes at the most northern latitudes were estimated to increase with over 500% for a four-degree centigrade increase protocol. The effect on aerosol and cloud formation is discussed and the cloud optical thickness is estimated to increase more than 2%.
Climate change is a global problem facing humanity and at the same time it is of anthropogenic origin [
Three examples of a positive feedback mechanism that result from an enhanced greenhouse effect with a subsequent increase in temperature are given by Mencuccini and Grace [
One example of a negative feedback mechanism was proposed by Kulmala et al. [
Schematic representation of the negative feedback mechanism proposed by Kulmala et al. [
Number 1 in Figure
In this study we extend the Kulmala et al. [
Schematic representation of the negative feedback mechanism proposed in this study. Marked in orange are modifications from the negative feedback mechanism proposed by Kulmala et al. [
To estimate changes of GPP upon a doubling of CO2 levels relative to the year 1975, we used the Lund-Potsdam-Jena (LPJ-GUESS) dynamic vegetation model [
To simulate GPP the dynamic vegetation model LPJ-GUESS was used [
The simulated area is located in Siberia in northern Russia. We simulate GPP for a specific geographical coordinate. This is done in a grid-box representing the selected area. Because of the spherical shape of Earth, the grid-box in the model is not rectangular, but rather a trapetzoid as a first simplification.
This study’s model domain is marked in Figure
(a) The permafrost region (white), (b) Siberia and the area under study. Each of the simulated red coordinates does correspond to the coordinates marked out in Figures
Three different emission protocols were used to model the response in forest growth. We assumed a doubling of the CO2 level compared to the year 1975 (330 ppm(v)) for all of the protocols after the simulated period of 200 years. The temperature increase corresponds to different levels of climate sensitivity, namely, 2, 3, or 4 degrees centigrade increase. The emission protocols are presented in Table
The emission protocols assumed in this study.
Emission protocol | Carbon dioxide level increase | Simulated years | Temperature increase |
---|---|---|---|
A | Doubling | 200 | 2 degrees centigrade |
B | Doubling | 200 | 3 degrees centigrade |
C | Doubling | 200 | 4 degrees centigrade |
In the LPJ-GUESS model larch forest is referred to as boreal needle summergreen (BNS) vegetation. In Siberia 46% of the forested area is made up by larch forest [
To see whether any new vegetation would be established, as a first survey we carried out LPJ-GUESS simulations over the permafrost region using the three emission protocols listed in Table
Vegetation establishing in permafrost regions after climate change.
Based on the three emission protocols in Table
In Figures
As can be seen from Figures
From Figures
In order to perform a back of the envelope calculation of the increase in VOC emissions in the sub-Arctic region that would follow forestation of the tundra, VOC emission rates of spruce, pine, and larch were used. Petersson [
Lind [
Ruuskanen et al. [
Examples of Pinaceae emissions of monoterpenes [
Examples of Pinaceae | Monoterpene emission ( |
---|---|
|
0.9 |
|
1.1 |
|
1.0 |
|
1.0 |
|
0.7 |
However, emissions of monoterpenes are temperature dependent. To account for this we used the following equation given by Kesselmeier and Staudt [
The factor for calculating the temperature dependency of monoterpene emissions was estimated from the average monthly temperature at longitudes E47.25, E92.25, and E152.25, respectively. The temperature increase according to the protocols A, B, and C (the increase in temperature due to climate change and thereby the deviation from 20°C) is compensated by an increase of the factor
Average monthly temperature at longitude E92.25.
BNE relative growth (%) in northern Russia for protocol A after 200-year simulation. The columns show the result for eastern longitudes and the rows for northern latitudes marked in Figure
BNS relative growth (%) in northern Russia for protocol A after 200-year simulation. The columns show the result for eastern longitudes and the rows for northern latitudes marked in Figure
BNE relative growth (%) in northern Russia for protocol B after 200-year simulation. The columns show the result for eastern longitudes and the rows for northern latitudes marked in Figure
BNS relative growth (%) in northern Russia for protocol B after 200-year simulation. The columns show the result for eastern longitudes and the rows for northern latitudes marked in Figure
BNE relative growth (%) in northern Russia for protocol C after 200-year simulation. The columns show the result for eastern longitudes and the rows for northern latitudes marked in Figure
BNS relative growth (%) in northern Russia for protocol C after 200-year simulation. The columns show the result for eastern longitudes and the rows for northern latitudes marked in Figure
The number of days is divided into bins of temperature. Each bin corresponds to a factor (
Temperature bins and factors for emission rates for different locations in the area under study (Figure
Temp. bin (°C) | Factor | Latitude (N) | Number of days for longitudes |
Latitude | Number of days for longitudes |
Latitude (N) | Number of days for longitudes |
---|---|---|---|---|---|---|---|
<0 | 0 | 69.25 | N/A | 69.25 | 270 | 69.25 | 240 |
66.25 | 210 | 66.25 | 240 | 66.25 | 240 | ||
63.25 | 210 | 63.25 | 210 | 63.25 | 210 | ||
60.25 | 150 | 60.25 | 210 | 60.25 | 240 | ||
57.25 | 150 | 57.25 | 180 | 57.25 | N/A | ||
|
|||||||
0–5 | 0.21 | 69.25 | N/A | 69.25 | 30 | 69.25 | 30 |
66.25 | 30 | 66.25 | 30 | 66.25 | 30 | ||
63.25 | 0 | 63.25 | 60 | 63.25 | 60 | ||
60.25 | 60 | 60.25 | 30 | 60.25 | 60 | ||
57.25 | 30 | 57.25 | 30 | 57.25 | N/A | ||
|
|||||||
5–10 | 0.30 | 69.25 | N/A | 69.25 | 60 | 69.25 | 60 |
66.25 | 60 | 66.25 | 60 | 66.25 | 30 | ||
63.25 | 60 | 63.25 | 0 | 63.25 | 0 | ||
60.25 | 60 | 60.25 | 30 | 60.25 | 60 | ||
57.25 | 30 | 57.25 | 60 | 57.25 | N/A | ||
|
|||||||
10–15 | 0.52 | 69.25 | N/A | 69.25 | 0 | 69.25 | 30 |
66.25 | 60 | 66.25 | 30 | 66.25 | 60 | ||
63.25 | 60 | 63.25 | 60 | 63.25 | 90 | ||
60.25 | 30 | 60.25 | 60 | 60.25 | 0 | ||
57.25 | 60 | 57.25 | 30 | 57.25 | N/A | ||
|
|||||||
>15 | 0.83 | 69.25 | N/A | 69.25 | 0 | 69.25 | 0 |
66.25 | 0 | 66.25 | 0 | 66.25 | 0 | ||
63.25 | 30 | 63.25 | 30 | 63.25 | 0 | ||
60.25 | 60 | 60.25 | 30 | 60.25 | 0 | ||
57.25 | 90 | 57.25 | 60 | 57.25 | N/A |
The number of days accounted for in each temperature bin is based on one month (30 days) for which the temperature is within the interval. On average, the model and the temperature profile are in agreement.
All days when the temperature exceeds 0°C have been assumed to emit monoterpenes. The emission rates by Ruuskanen et al. [ for A: 14.5 Tg y−1, for B: 16.4 Tg y−1, for C: 17.0 Tg y−1.
As a comparison, Laothawornkitkul et al. [
The number of CCN available for cloud droplet activation is hard to estimate because the process of condensational growth of the newly formed particles involving monoterpenes is not fully understood [
In the boreal forest Kerminen et al. [
Kulmala et al. [
Firstly, the increase in average temperature and the increase of CO2 in the atmosphere as a result of climate change not only contribute to an increase of numbers of trees in the forests, as stated by Kulmala et al. [
Secondly, the effect on surface tension and thereby the lowering of supersaturation of water vapour needed for condensational growth by water vapour of droplets according to Köhler theory [
Thirdly, monoterpenes evaporated from boreal trees react quickly with oxidizing reactants like ozone and hydroxyl radicals producing low-volatile substances such as
In the high Arctic summer there are few of available particles for cloud droplet formation, since the air is clean with limited influences from manmade activities [
The fourth issue is the most important. Kulmala et al. [
In any kind of attempt to simulate feedback mechanisms there are both simplifications made and uncertainties to consider. For Step 1 in Figure
For the calculations of increase in emissions of monoterpenes from forest growth, the resolution of the simulated area is of vital importance. The more accurate the resolution of the calculated locations is, the higher the degree of information would be. To meet the need for highly resolved simulations we use the 50 km × 50 km horizontal grid resolution provided by the LPJ-GUESS model. By this we increase the resolution by 5 to 10 times relative to using atmospheric GCMs. However, the uncertainties in the assumptions used in the simulations described above limit the level of accuracy in the calculations. An even higher resolution would therefore not favor the final results.
The temperature increase that we set to 2, 3, and 4 degrees centigrade is corresponding to an increase globally in the LPJ-GUESS model evenly distributed. This is, however, not the case since we can see an amplification of the warming in the Arctic region and a global mean of 2-degree centigrade increase could actually correspond to a higher temperature in the Arctic region. This effect is not considered in our simulations. This suggests that the enhancement of the forests-aerosol-climate negative feedback mechanism is most likely even more pronounced at a lower global temperature increase.
In this study model simulations of the vegetation in northern Russia were carried out for the climate of the year 1975 as a reference and for three possible future projections: two-, three-, and four-degree centigrade increase of mean global temperature and with a doubling of the atmospheric carbon dioxide concentration over 200 years. The simulations were carried out using the LPJ-GUESS model. The growth of the forest was converted to emissions of monoterpenes based on literature values and compensating for the number of days of sunlight and to a temperature factor.
The results show the following. There was a growth of the boreal forest both for spruce and pine (in the central and western parts of the forest area) and larch (in the north-eastern part of the forest), as well as new forestation of previously wastelands of the tundra. Based on the simulations the boreal forest will move northward as a result of global warming. The emissions of monoterpenes at the most northern latitudes were estimated to increase with close to 500% for a four-degree centigrade increase protocol. The likelihood for organic vapours and particles to be transported northward over the central Arctic Ocean is highest for this northern latitude at the same time as the projected increase in precursors for particle formation is huge. Kulmala et al. [
The authors have no conflict of interests to declare.