To concatenate with BSD Join
join works for me here:

pse@Mithos:~/t$ join -1 1 -2 1 t1 t2
Model Decreasing Constant Increasing Decreasing Constant Increasing
2025 75.5 82.5 89.5 2.5 2.5 2.5
2035 65.5 82.5 102 2.5 2.5 5
2050 49.5 82.5 112 7.5 7.5 7.5


In case there are non-matching header lines in each file you need to get rid of them first, otherwise the joining will fail.

pse@Mithos:~/t$ join -1 1 -2 1 &lt;(grep -v Year t1) &lt;(grep -v Model t2)
2025 75.5 82.5 89.5 2.5 2.5 2.5
2035 65.5 82.5 102 2.5 2.5 5
2050 49.5 82.5 112 7.5 7.5 7.5