Topic sentences for causes:
One of the main reasons for X is Y
X can be another contributing factor
X is the main/another driver of Y

Topic sentences for solutions:
One effective measure is …
X is another solution
Another approach is …

Topic sentences for effects:
X is mostly affected by this
Y has a substantial effect on X
the impact of X can also be seen on Y

Include a bridging sentence between the paragraphs:
However, there are several ways by which the problem can be tackled.
Despite this, the problem can be alleviated(=mitigate/ease) in a number of ways.