diff --git a/src/variables.jl b/src/variables.jl
index cca72f8..e7ae566 100644
--- a/src/variables.jl
+++ b/src/variables.jl
@@ -15,7 +15,7 @@ const PREDEFINED_EBM_VARIABLES = @variables begin
     (ε(t) = 0.5),        [bounds = (0.0, 1.0), description = "planetary effective emissivity, unitless"]
     (C(t) = 0.6744),     [bounds = (0.0, 1.0), description = "cloud fraction, unitless"]
     (ℓ(t) = 0.8),        [bounds = (0.0, 1.0), description = "sine of latitude of ice-line"]
-    (CO2(t) = 400),      [bounds = (200.0, 1800.0), description = "CO2 concentration, in ppm"]
+    (CO2(t) = 400.0),    [bounds = (200.0, 1800.0), description = "CO2 concentration, in ppm"]
     # Observables that can never be dynamic variables and hence have no default value:
     (OLR(t)), [description = "outgoing longwave radiation"]
     (ASR(t)), [description = "absorved shortwave radiation"]
@@ -28,16 +28,28 @@ export T, S, f, α, α_ice, α_cloud, ΔT, ε, ℓ, C, CO2, OLR, ASR
     physically_plausible_limits(x)
 
 Return a tuple (min, max) of plausible limiting values for the variable `x`.
+If the variable does not have defined `bounds`, then the default value ± 20% is used.
+If there is no default value, a heuristic is tried, and an error is thrown if it fails.
 """
 function physically_plausible_limits(var)
     if ModelingToolkit.hasbounds(var)
         return getbounds(var)
     elseif !isnothing(default_value(var))
         return (0.8default_value(var), 1.2default_value(var))
+    else
+        return physically_plausible_limits(string(ModelingToolkit.getname(var)))
+    end
+end
+
+function physically_plausible_limits(var::String)::Tuple{Float64, Float64}
+    if var[1] == 'T'
+        return (200, 350)
+    elseif var[1] == 'α' || var[1] == 'ε' || var[1] == 'C'
+        return (0, 1)
     else
         error("""
         Unpsecified plausible physical limits for $(var): it has no defined bounds or
         a default variable. You need to redefine the variable to have either of the two.
         """)
     end
-end
+end
\ No newline at end of file